md: check return code of read_sb_page

Function read_sb_page may return ERR_PTR(...). Check for it.
Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/bitmap.c

@@ -1000,7 +1000,8 @@ static int bitmap_init_from_disk(struct bitmap *bitmap, sector_t start)
 				page = bitmap->sb_page;
 				offset = sizeof(bitmap_super_t);
 				if (!file)
-					read_sb_page(bitmap->mddev,
+					page = read_sb_page(
+						bitmap->mddev,
 						bitmap->mddev->bitmap_info.offset,
 						page,
 						index, count);