mm:page-writeback: use folio_next_index() helper in writeback_iter()

Simplify code pattern of 'folio->index + folio_nr_pages(folio)' by using
the existing helper folio_next_index().

Link: https://lkml.kernel.org/r/20240821063112.4053157-1-liyuesong@vivo.comSigned-off-by: Yuesong Li <liyuesong@vivo.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page-writeback.c

@@ -2612,7 +2612,7 @@ struct folio *writeback_iter(struct address_space *mapping,
 
 done:
 	if (wbc->range_cyclic)
-		mapping->writeback_index = folio->index + folio_nr_pages(folio);
+		mapping->writeback_index = folio_next_index(folio);
 	folio_batch_release(&wbc->fbatch);
 	return NULL;
 }