afs: fix the afs_dir_get_folio return value

Keep returning NULL on failure instead of letting an ERR_PTR escape to
callers that don't expect it.

Link: https://lkml.kernel.org/r/20230503154526.1223095-2-hch@lst.de
Fixes: 66dabbb ("mm: return an ERR_PTR from __filemap_get_folio")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: David Howells <dhowells@redhat.com>
Tested-by: David Howells <dhowells@redhat.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Christoph Hellwig

fs/afs/dir_edit.c

@@ -115,11 +115,12 @@ static struct folio *afs_dir_get_folio(struct afs_vnode *vnode, pgoff_t index)
 	folio = __filemap_get_folio(mapping, index,
 				    FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
 				    mapping->gfp_mask);
-	if (IS_ERR(folio))
+	if (IS_ERR(folio)) {
 		clear_bit(AFS_VNODE_DIR_VALID, &vnode->flags);
-	else if (folio && !folio_test_private(folio))
+		return NULL;
+	}
+	if (!folio_test_private(folio))
 		folio_attach_private(folio, (void *)1);
-
 	return folio;
 }