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>

fs/ufs/util.c

@@ -245,7 +245,7 @@ struct folio *ufs_get_locked_folio(struct address_space *mapping,
 {
 	struct inode *inode = mapping->host;
 	struct folio *folio = filemap_lock_folio(mapping, index);
-	if (!folio) {
+	if (IS_ERR(folio)) {
 		folio = read_mapping_folio(mapping, index, NULL);
 
 		if (IS_ERR(folio)) {