mm: fix arithmetic for bdi min_ratio

Since now bdi->min_ratio is part per million, fix the wrong arithmetic. 
Otherwise it will fail with -EINVAL when setting a reasonable min_ratio,
as it tries to set min_ratio to (min_ratio * BDI_RATIO_SCALE) in
percentage unit, which exceeds 100% anyway.

    # cat /sys/class/bdi/253\:0/min_ratio
    0
    # cat /sys/class/bdi/253\:0/max_ratio
    100
    # echo 1 > /sys/class/bdi/253\:0/min_ratio
    -bash: echo: write error: Invalid argument

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 8021fb3 ("mm: split off __bdi_set_min_ratio() function")
Signed-off-by: Jingbo Xu <[email protected]>
Reported-by: Joseph Qi <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Stefan Roesch <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/page-writeback.c

@@ -692,7 +692,6 @@ static int __bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ra
 
 	if (min_ratio > 100 * BDI_RATIO_SCALE)
 		return -EINVAL;
-	min_ratio *= BDI_RATIO_SCALE;
 
 	spin_lock_bh(&bdi_lock);
 	if (min_ratio > bdi->max_ratio) {