[PATCH] fat: Fix fat_ra_init() for data clusters == 0

[OGAWA Hirofumi]

If data clusters == 0, fat_ra_init() calls the ->ent_blocknr() for the
cluster beyond ->max_clusters.

This checks the limit before initialization to suppress the warning.

Reported-by: syzbot+756199124937b31a9b7e@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
---
 fs/fat/fatent.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index bbfe18c..f7e3304 100644
--- a/fs/fat/fatent.c	2020-07-11 19:58:41.903092419 +0900
+++ b/fs/fat/fatent.c	2020-07-11 19:58:51.545948758 +0900
@@ -657,6 +657,9 @@ static void fat_ra_init(struct super_blo
 	unsigned long ra_pages = sb->s_bdi->ra_pages;
 	unsigned int reada_blocks;
 
+	if (fatent->entry >= ent_limit)
+		return;
+
 	if (ra_pages > sb->s_bdi->io_pages)
 		ra_pages = rounddown(ra_pages, sb->s_bdi->io_pages);
 	reada_blocks = ra_pages << (PAGE_SHIFT - sb->s_blocksize_bits + 1);
_