f2fs: fix the size value in __check_sit_bitmap

commit 008396e1b026b3873091b555b808155da7d9d18f upstream.

The current size value is not correct and will miss bitmap check.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>