DESCRIPTION DEVICE FOR PREVENTING WINDING-UP OF SHEET METAL IN CONTINUOUS HOT-DIPPING BATH TECHNICAL FIELD The present invention relates to a system for prevention of stirred up dross provided with flow regulating members for suppressing stir up and deposition on the plating surface of bottom dross and other solid particles precipitated or floating in a plating bath in the continuous hot-dip plating process of steel sheet or other metal sheet. BACKGROUND ART Various types of hot dip plated metal sheet have been developed and commercialized up to now. Among these, hot dip galvanized steel sheet is spreading widely as a material for automobiles, buildings, the electric home appliances, etc. due to its superior corrosion resistance and economy. The present invention may be applied to not only hot dip galvanization, but also aluminum plating, tin plating, and other various types of hot-dip plating bath, but the case of the most general hot dip galvanization system for steel sheet will be taken as an example and explained below. When continuously producing hot dip galvanized steel sheet, the method of dipping and moving the steel sheet in a hot-dip plating bath to plate it has been generally used. At this time, it is known that the solid particles of impurities precipitated and deposited at the bottom of the hot-dip plating bath, for example, bottom dross, are stirred up along with the movement of the steel sheet during the plating treatment and stick to the steel sheet plated surface to thereby detract from the appearance of the plated steel sheet. Various measures are being tried out on the work site to deal with this stirred up bottom dross in hot-dip plating bath, but no complete solution has yet been found. FIG. 9 shows the outlines of a generally used continuous hot dip galvanization system of steel sheet. In the continuous hot dip galvanization system shown in FIG. 9, after a steel sheet 1 is annealed by an annealing furnace (not shown), it passes through a snout 2 and enters a hot dip galvanization bath 3. The introduced steel sheet is changed in direction to face upward by a sink roll 4 provided inside the hot dip galvanization bath 3, is corrected for warping by support rolls 5, then is pulled out from a plating bath surface 6. Next, the two sides of the hot dip galvanized plated steel sheet 1' are blown with wiping gas from gas wiping nozzles 7 to adjust the amount of coating weight Furthermore, the plated steel sheet 1' is passed through a vibration dampening system 8 for correcting its shape and suppressing steel sheet vibration, and then treated to alloy the plating as needed in a galvannealing furnace 9. Inside the hot dip galvanization bath, Fe elutes from the steel sheet into the hot dip galvanization bath whereby particulate and granular substances comprised of Fe-Zn intermetallic compounds, that is, so-called "dross", are produced. In this dross, the part mainly comprised of FeZn-/ has a larger specific gravity than the molten zinc, so precipitates and deposits on the plating bath bottom. In general, this is called "bottom dross" (see 10 in FIG. 9) - Bottom dross starts to be stirred up due to the accompanying flow caused by movement of the steel sheet-circling the sink roll in the galvanization bath and finally sticks to the surface of the plated steel sheet to cause poor appearance of the plated steel sheet. In particular, the bottom dross is caught and pressed at the part where the sink roll or support roll and steel sheet contact each other and remains on the plated steel sheet thereby becoming a cause aggravating the poor appearance when press forming the plated steel sheet into the final product. In particular, on recent work sites, attempts are being made to raise the processing rate of steel sheet to improve production capabilities. Along with this, in the plating bath, the agitation becomes stronger, and the amount of elution of Fe, the cause of formation of dross, increases, and the bottom dross is stirred up much more vigorously. Further, on the other hand, customers are becoming tougher regarding the quality of appearance of the plated steel sheet they seek. There is therefore pressure on the work floor for solving the problem of stirred up bottom dross. To solve this problem, various proposals have been made in the past. For example, Japanese Patent Publication (B2) No. 6-21331 and Japanese Utility Model Publication (U) No. 5-38045 propose a method of suppressing stirred up bottom dross comprising providing a covering plate covering the entire cylinder length of the sink roll and suppressing flow in the plating bath between the sink roll and plating tank bottom and forming a space in which bottom dross deposits below this covering plate. Further, Japanese Patent Publication (A) No. 6-158253 proposes a continuous hot dip galvanization system providing a multi-hole plate suppressing flow motion in the bath between the sink roll and plating tank bottom. Furthermore, Japanese Patent Publication (A) No. 2001-140050 proposes a system for prevention of stirred up bottom dross characterized by providing two plate-shaped members of lengths corresponding to 20 to 40% of the sink roll cylinder length away from the sink roll surface from the two ends of the sink roll toward the center. However, with these proposals, as explained later, it is difficult to completely solve the problem of stirred up bottom dross. For stirred up bottom dross in hot dip galvanization baths, in the past it had mainly been considered that the tangential direction force caused along with rotation of the sink roll (see 11 in FIG. 9) caused the bottom dross deposited at the bottom near the sink roll to be stirred up. However, the inventors worked to study the phenomenon of stirred up bottom dross by running 3D flow motion analysis on the inside of hot dip galvanization baths. As a result, they discovered that the flow accompanying the steel sheet becomes strong at the part narrowed by the sink roll. That is, jet flow occurring at the sides of the contact part of the sink roll moves strongly toward the bottom of the sides in the hot dip galvanization bath, so the inventors discovered that the bottom dross deposited at the bottom of the hot dip galvanization bath was stirred up. When the steel sheet is a broad width material, as shown in FIG. 10(a), the jet flow occurring at the sides of the contact part of the sink roll causes a force to act on the bottom dross stirring up the bottom dross from near the front center of the sink roll 4 (see A in the figure). Further, when the steel sheet is a narrow width material, as shown in FIG. 10(b), a force acts on the bottom dross stirring up the bottom dross between the sink roll 4 and the side walls of the hot dip galvanization bath 3 (see B in the figure). In each case, as a result, the bottom dross is stirred up inside the plating bath in a manner drawing a circle in the vertical direction and enters a floating state. The inventors elucidated the mechanism whereby the bottom dross enters a floating state and has a detrimental effect on plating of steel sheet. Assuming this mechanism, the prior art has the following problems: First, the method disclosed in Japanese Patent Publication (B2) No. 6-21331 and Japanese Utility Model Publication (U) No. 5-38045 can effectively prevent stirred up bottom dross by the provision of a covering plate for bottom dross trying to flow in a tangential direction of the circumference due to rotation of the sink roll, but no covering measure or flow regulating measure is taken against the wall surface flow occurring at the two side surfaces of the sink roll, so the stirred up bottom dross cannot be sufficiently suppressed. Second, the apparatus disclosed in Japanese Patent Publication (A) No. 6-158253 does not provide any means for solving the problem of the wall surface flow occurring at the two side surfaces of the sink roll, so the effect of suppressing stirred up bottom dross is not sufficient. Further, in the above apparatus, the multi-hole flow regulating plate is provided for substantially the entire sink roll in the width direction, so turbulence occurs between the sink roll and multi-hole flow regulating plate, bottom dross sticks to the surface of the steel sheet not contacting the sink roll, and bottom dross is liable to deposit on the multi-hole flow regulating plate. Furthermore, in the above apparatus, there is the problem that at the time of replacement of the sink roll etc., the work of threading the steel sheet through the sink roll and rest of the plating apparatus becomes complicated. Third, in the apparatus disclosed in Japanese Patent Publication (A) No. 2001-140050, two covering plates are set at the two sides of the sink roll in a state separated from each other, so the problem at the time of replacement of the sink roll etc. is solved, but no measure is taken for regulating the wall surface flow occurring at the two side surfaces of the sink roll, so stirred up bottom dross cannot be completely suppressed. Further, in the above apparatus, when the distance between the sink roll and the plate members is large, the bottom dross deposits on the plate members, so when changing the steel sheet from a narrow width material to a broad width material, the stirring up of the deposited dross is aggravated. Conversely, when the distance between the sink roll and plate members is small, a strong flow including bottom dross concentrates at this small distance, so the bottom dross is scattered throughout the plating bath and the bottom dross is liable to be caught between the sink roll or support roll and the steel sheet. DISCLOSURE OF THE INVENTION The present invention has as its object the provision of a system able to prevent impurities precipitated and deposited at the bottom of a hot dip galvanization bath, that is, bottom dross, in the process of continuous hot dip galvanization of steel sheet, from being stirred up and sticking on the plated surface of the steel sheet along with movement of the steel sheet during the plating treatment regardless of the width of the steel sheet. The present invention was made to solve the above problem and has as its gist the following: (1) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet characterized by placing flow regulating members partially separated from each other under a sink roll at the two side wall surfaces of a hot-dip plating bath which axial ends of the sink roll face so that said members contact the wall surfaces and thereby suppressing the flow of hot dip metal ascending or descending along the wall surfaces. (2) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet characterized by placing flow regulating members at the two side wall surfaces of a hot-dip plating bath which axial ends of the sink roll face so that said members contact the wall surfaces and so that parts of said members are positioned at positions of a distance from the hot-dip plating bath bottom exceeding 0.8 time a distance between said bottom and a bottom end of the sink roll and thereby suppressing the flow of hot dip metal ascending or descending along the wall surfaces. (3) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet characterized by placing flow regulating members at a front wall surface and/or rear wall surface of a hot-dip plating bath so that said members contact said wall surfaces and thereby suppressing the flow of hot dip metal ascending or descending along the wall surfaces. (4) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet as set forth in (3) characterized by placing flow regulating members at the two side wall surfaces of said hot-dip plating bath so that said members contact said wall surfaces and thereby suppressing the flow of hot dip metal ascending or descending along the wall surfaces. (5) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet as set forth in any of the above (1) to (4) characterized in that a width dimension W of the flow regulating members placed at the side wall surfaces of said hot-dip plating bath is shorter than a distance X from said side wall surfaces to ends of the steel sheet and longer than a distance Z from said side wall surfaces to support members of the sink roll. (6) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet as set forth in any of the above (1) to (5) characterized in that a depth dimension L of a flow regulating member set at a side wall surface of said hot-dip plating bath is longer than 0.7 time the sink roll diameter and shorter than a depth dimension Y of the inside of the hot-dip plating bath. (7) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet as set forth in any of the above (1) to (6) characterized in that said flow regulating member is provided with a plurality of holes and the total of the areas of the holes is 10 to 70% of the total area of said flow regulating member. (8) A system for prevention of stirred up dross in continuous hot-dip plating bath of a metal sheet as set forth in any of the above (1) to (7) characterized in that said flow regulating member is provided with a plurality of holes and has an average area per hole of 1.2xl04 mm2 or less. According to the present invention, when performing the continuous hot dip galvanization treatment in the state of a high processing rate, it becomes possible to reliably suppress the stirring up of bottom dross precipitated and deposited in the plating bath more than in the past and thereby greatly reduce the sticking of bottom dross on the plated steel sheet. Further, according to the present invention, when replacing the sink roll etc., the work of threading the steel sheet through the sink roll and rest of the plating system can be performed easier than in the past. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a first embodiment of the present invention. FIG. 2 is a view showing the modes of bath flow occurring in the first embodiment of the present invention, (a) shows the case of a broad width material, while (b) shows the case of a narrow width material. FIG. 3 shows the mode of placement of a multi-hole flow regulating plate, (a) shows a mode where the multi-hole flow regulating plate is suspended via a support member, while (b) shows the mode where the multi-hole flow regulating plate is propped up via a support member FIG. 4 is a view showing a second embodiment of the present invention. FIG. 5 is a view showing a third embodiment of the present invention. FIG. 6 is a view showing a fourth embodiment of the present invention. FIG. 7 is a view showing a fifth embodiment of the present invention. FIG. 8 is a view for explaining the method of determination of the dimensions of a flow regulating member placed at the side wall surface, (a) shows a side view of a continuous hot dip galvanization system, while (b) shows a front view. FIG. 9 is a view showing an outline of a general continuous hot dip galvanization system. FIG. 10 is a view showing the aspect of bath flow occurring in the system shown in FIG. 9. (a) shows the case of a broad width material, while (b) shows the case of a narrow width material. BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will be explained first. The mode of bath flow in the case of using flow regulating members provided with a large number of holes, that is, "multi-hole flow regulating plates", is shown in FIG. 2. When compared with FIG. 10 showing the state where no flow regulating members are provided, the direction of bath flow is almost unchanged, but due to the provision of the multi-hole flow regulating plates, an action and effect of reducing the flow rate at the wall surfaces are obtained. These action and effect enable the stirred up bottom dross to be kept in a low region of the hot dip galvanization bath. Further, due to this action and effect, the amount of bottom dross being caught in the steel sheet can be reduced. Note that the effect due to the provision of the multi-hole flow regulating plates is believed to be mainly effective against bottom dross being caught up at the support rolls in the case of a broad width material and to be mainly effective against bottom dross being caught up in the sink roll in the case of a narrow width material. In the present invention, the flow regulating members are provided in the plating bath, as shown in FIG. 1, so that the flow regulating members 22 contact the two side wall surfaces 21 in the plating bath 3. The "contact" referred to in the present invention does not mean the state where the flow regulating members 22 are directly attached to the side wall surfaces 21 of the plating bath. The "contact", as shown in FIG. 3 (a), also means the state of attaching a flow regulating member 22 to a support member 23 suspended from above and, further, as shown in FIG. 3 (b), the state of attaching a flow regulating member 22 to a support member 23 propped up at the bottom of the plating bath and other cases where there is a slight clearance between the side wall surface 21 and flow regulating member 22. For example, in actual operation, when stopping production and performing cleaning work to remove the bottom dross deposited in the plating bath, sometimes having the flow regulating members 22 fastened to the two side wall surfaces 21 would interfere with the cleaning work. On the other hand, as shown in FIGS. 3(a) and (b), if suspending the flow regulating member 22 by support members 23 like pipe frames or propping up the flow regulating members 22 on support members 23, attachment/detachment of the flow regulating members 22 would become easy and cleaning work could be easily performed. Even if the flow regulating members 22 do not completely contact the two side wall surfaces 21, in practice the effect of reducing the side wall surface flow can be expected. Note that as another method of attaching the flow regulating members 22, attachment to the support members of the sink roll to make integral members is also possible. Further, in the present invention, the flow regulating members are provided in the plating bath so that, as shown in FIG. 1, at least parts of the flow regulating members 22 in the plating bath 3 are separated from each other under the sink roll. That is, this means the state where there are no flow regulating members near the center of the sink roll and the space is open, in other words, the members of the pair or pairs of flow regulating members contacting the two side wall surfaces are arranged separated from each other across a distance. The reason for this is that if placing the flow regulating members along the entire cylinder length of the sink roll, the dross floating near the center of the sink roll is liable to precipitate and deposit on the flow regulating members and later be stirred up. Near the center of the sink roll, the flow of the molten metal is slower than near the two ends, so even if there are no flow regulating members there, there is little risk of the bottom dross being stirred up. Further, if flow regulating members are provided extending across the bottom of the sink roll near its center, the work of attaching the front end of the steel sheet to the sink roll when starting the plating operation, the so-called "threading" work, will become complicated. Second and third embodiments of the present invention will be explained based on FIG. 4 and FIG. 5. FIG. 4 and FIG. 5 are views showing modes of placement of flow regulating members at the two side wall surfaces of a hot-dip plating bath which the axial ends of the sink roll face so that parts of the flow regulating members are positioned at locations of a distance from the bottom of the hot-dip plating bath of over 0.8 time the distance between said bottom and the bottom end of the sink roll. FIG. 4 shows a mode of placing flow regulating members with step differences so that the parts contacting the side wall surfaces are positioned higher than the bottom end of the sink roll. In the present invention, the flow regulating members are preferably provided at positions lower than the bottom end of the sink roll, more preferably positions 0.8 times or less the distance to the bottom end. However, as shown in FIG. 4, even if parts of the flow regulating members are provided at positions over 0.8 times the distance, the ascending flow along the wall surfaces can be effectively suppressed. The embodiment shown in FIG. 5 is also based on an idea similar to the embodiment shown in FIG. 4, but differs in the point that the flow regulating members are arranged at the support members of the sink roll. As explained above, the flow regulating members of the present invention are designed to capture the ascending flow or descending flow along the wall surfaces and keep down the flow rate, so the heights of the flow regulating members do not have to be single heights. The flow regulating members of the present invention function to capture and regulate the ascending flow and descending flow at different height positions. The fourth and fifth embodiments of the present invention will be explained based on FIG. 6 and FIG. 7. FIG. 6 and FIG. 7 are views showing the state of placing flow regulating members in the hot-dip plating bath at the front wall surface and/or rear wall surface in addition to the side wall surfaces. If the speed of movement of the steel sheet and the speed of rotation of the sink roll rise, a flow of hot dip metal will occur at both the front (exit side region of steel sheet) and rear (entry side region of steel sheet) of the hot-dip plating bath and may stir up the bottom dross. Further, along with suppressing the flow at the side parts by the flow regulating members, the blocked flow of hot dip metal has nowhere to go and is liable to collect at the front or rear forming new stirring up loops. These embodiments place flow regulating members at least at the front wall surface and/or rear wall surface and aim at suppressing the flow of molten metal from numerous directions. The flow regulating members used in the present invention are not limited to the above multi-hole flow regulating plates. Various forms of members may be used. For example, block-shaped members, wadding-shaped members, net-shaped members, members comprised of cages in which pellets are filled, and other members having the effect of reducing the wall surface flow rate may be freely used. Further, the flow regulating members of the present invention do not necessarily have to be placed horizontally or flat. To prevent bottom dross from depositing on the flow regulating members, they may be placed at a slant or the members may be placed after bending them in advance. By providing a large number of holes in the flow regulating members, it becomes possible to reduce the flow rate at the wall surfaces and still allow the passage of particle-shaped bottom dross. As a result, the amount of bottom dross depositing on the flow regulating members is reduced, so it is possible to reduce newly stirred up bottom dross. On the other hand, in the case of hole-less flow regulating members, there is no such passage effect, but these are superior over flow regulating members provided with holes in the point of blocking the wall surface flow. Note that the flow regulating members may be suitably provided with holes in accordance with need. The flow regulating members of the present invention are not limited to ones with or without holes. However, when providing holes in the flow regulating members, the total area of the holes is preferably made 10 to 70% (aperture rate) of the total area of the flow regulating members and the average area per hole is preferably not more than 1.2xl04 mm2. The aperture rate is more preferably 30 to 60%. If using flow regulating members with an aperture rate of 30 to 60%, a remarkable effect of keeping bottom dross from being stirred up can be obtained. Flow regulating members with an aperture rate of less than 10% are poor in the ability to pass bottom dross and are susceptible to bottom dross depositing on the flow regulating members. In particular, deposition of bottom dross is aggravated when processing broad width materials. On the other hand, flow regulating members with an aperture rate exceeding 70% are inferior in ability to reduce the wall surface flow rate and cannot effectively suppress stirred up bottom dross. Further, if the average area per hole exceeds 1.2xl04 mm2, it becomes difficult to uniformly reduce the wall surface flow rate, so this is not preferred. Note that the lower limit of the average area per hole is not particularly set, but the size of the bottom dross is usually on the p,m order to several mm or so, so the area of the holes should be any area of an extent enabling easy passage of the bottom dross, for example, 10 mm2 or more. The shape of the holes is also not particularly limited. Members provided with regular circular holes like punched metal, members made of metal mesh, etc. may be suitably selected as flow regulating members. The dimensions of the flow regulating members of the present invention should be dimensions enabling effective regulation of the ascending flow or descending flow along the wall surfaces and are suitably determined in accordance with the dimensions of the hot-dip plating bath facility used. The method of determining the dimensions of the flow regulating members placed at the side wall surfaces will be explained with reference to FIG. 8. The width dimension W of the flow regulating members is set shorter than the distance X from the side wall surfaces to the ends of the steel sheet and longer than the distance Z from the side wall surfaces to the sink roll support members. When W>X, the amount of bottom dross depositing on the flow regulating members increases and the freguency of dross sticking to the steel sheet (bottom dross sticking rate) rises. Further, at the time of replacing the sink roll, the work of threading the steel sheet is liable to be obstructed. On the other hand, when WY, it is physically impossible for the plating bath to house the flow regulating plates. Therefore, the depth dimension L of the flow regulating members preferably satisfies 0.7D