mm: Add folio_flush_mapping()

mm: Add folio_flush_mapping()

This is the folio equivalent of page_mapping_file(), but rename it
to make it clear that it's very different from page_file_mapping().
Theoretically, there's nothing flush-only about it, but there are no
other users today, and I doubt there will be; it's almost always more
useful to know the swapfile's mapping or the swapcache's mapping.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>