mm: split off __bdi_set_max_ratio() function

This splits off __bdi_set_max_ratio() from bdi_set_max_ratio().
__bdi_set_max_ratio() will also be called from bdi_set_max_bytes(),
which will be introduced in the next patch.

Link: https://lkml.kernel.org/r/20221119005215.3052436-7-shr@devkernel.ioSigned-off-by: Stefan Roesch <shr@devkernel.io>
Cc: Chris Mason <clm@meta.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page-writeback.c

@@ -692,14 +692,10 @@ int bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ratio)
 	return ret;
 }
 
-int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
+static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio)
 {
 	int ret = 0;
 
-	if (max_ratio > 100)
-		return -EINVAL;
-	max_ratio *= BDI_RATIO_SCALE;
-
 	spin_lock_bh(&bdi_lock);
 	if (bdi->min_ratio > max_ratio) {
 		ret = -EINVAL;
@@ -711,6 +707,14 @@ int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned max_ratio)
 
 	return ret;
 }
+
+int bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ratio)
+{
+	if (max_ratio > 100)
+		return -EINVAL;
+
+	return __bdi_set_max_ratio(bdi, max_ratio * BDI_RATIO_SCALE);
+}
 EXPORT_SYMBOL(bdi_set_max_ratio);
 
 u64 bdi_get_max_bytes(struct backing_dev_info *bdi)