gfs2: increase usage of folio_next_index() helper

Simplify code pattern of 'folio->index + folio_nr_pages(folio)' by using
the existing helper folio_next_index().
Signed-off-by: Minjie Du <duminjie@vivo.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/aops.c

@@ -272,8 +272,7 @@ static int gfs2_write_jdata_batch(struct address_space *mapping,
 				 * not be suitable for data integrity
 				 * writeout).
 				 */
-				*done_index = folio->index +
-					folio_nr_pages(folio);
+				*done_index = folio_next_index(folio);
 				ret = 1;
 				break;
 			}