f2fs: 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>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -3232,8 +3232,7 @@ result:
 					}
 					goto next;
 				}
-				done_index = folio->index +
-					folio_nr_pages(folio);
+				done_index = folio_next_index(folio);
 				done = 1;
 				break;
 			}