fix ufs_get_locked_folio() breakage

filemap_lock_folio() returns ERR_PTR(-ENOENT) if the thing is not
in cache - not NULL like find_lock_page() used to.

Fixes: 5fb7bd50 "ufs: add ufs_get_locked_folio and ufs_put_locked_folio"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>