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.

Link: https://lkml.kernel.org/r/20230802151406.3735276-5-willy@infradead.org


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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>