mm: fix arithmetic for max_prop_frac when setting max_ratio

Since now bdi->max_ratio is part per million, fix the wrong arithmetic for
max_prop_frac when setting max_ratio.  Otherwise the miscalculated
max_prop_frac will affect the incrementing of writeout completion count
when max_ratio is not 100%.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: efc3e6a ("mm: split off __bdi_set_max_ratio() function")
Signed-off-by: Jingbo Xu <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Stefan Roesch <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: lostjeffle

mm/page-writeback.c

@@ -728,7 +728,8 @@ static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ra
 		ret = -EINVAL;
 	} else {
 		bdi->max_ratio = max_ratio;
-		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100;
+		bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) /
+						(100 * BDI_RATIO_SCALE);
 	}
 	spin_unlock_bh(&bdi_lock);