mm/filemap: Add folio private_2 functions

end_page_private_2() becomes folio_end_private_2(),
wait_on_page_private_2() becomes folio_wait_private_2() and
wait_on_page_private_2_killable() becomes folio_wait_private_2_killable().

Adjust the fscache equivalents to call page_folio() before calling these
functions to avoid adding wrappers.  Ends up costing 1 byte of text
in ceph & netfs, but the core shrinks by three calls to page_folio().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>