mm/filemap: 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() in filemap_get_folios_contig().

Link: https://lkml.kernel.org/r/20231107024635.4512-1-duminjie@vivo.com


Signed-off-by: Minjie Du <duminjie@vivo.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>