f2fs: fix null reference error when checking end of zone

This patch fixes a potentially null pointer being accessed by
is_end_zone_blkaddr() that checks the last block of a zone
when f2fs is mounted as a single device.

Fixes: e067dc3c ("f2fs: maintain six open zones for zoned devices")
Signed-off-by: Daejun Park <daejun7.park@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -925,6 +925,7 @@ int f2fs_merge_page_bio(struct f2fs_io_info *fio)
 #ifdef CONFIG_BLK_DEV_ZONED
 static bool is_end_zone_blkaddr(struct f2fs_sb_info *sbi, block_t blkaddr)
 {
+	struct block_device *bdev = sbi->sb->s_bdev;
 	int devi = 0;
 
 	if (f2fs_is_multi_device(sbi)) {
@@ -935,8 +936,9 @@ static bool is_end_zone_blkaddr(struct f2fs_sb_info *sbi, block_t blkaddr)
 			return false;
 		}
 		blkaddr -= FDEV(devi).start_blk;
+		bdev = FDEV(devi).bdev;
 	}
-	return bdev_is_zoned(FDEV(devi).bdev) &&
+	return bdev_is_zoned(bdev) &&
 		f2fs_blkz_is_seq(sbi, devi, blkaddr) &&
 		(blkaddr % sbi->blocks_per_blkz == sbi->blocks_per_blkz - 1);
 }