mm/filemap: use folio_next_index() for start

mm/filemap: use folio_next_index() for start

Use folio_next_index() instead of open-coding folio->index +
folio_nr_pages(folio) when updating @start in filemap_get_folios_contig(),
filemap_get_folios_tag(), and filemap_get_folios_dirty().

Link: https://lore.kernel.org/20260601110425.44784-1-tanze@kylinos.cn
Signed-off-by: tanze <tanze@kylinos.cn>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>