mm,jfs: move write_one_page/folio_write_one to jfs

The last remaining user of folio_write_one through the write_one_page
wrapper is jfs, so move the functionality there and hard code the
call to metapage_writepage.

Note that the use of the pagecache by the JFS 'metapage' buffer cache
is a bit odd, and we could probably do without VM-level dirty tracking
at all, but that's a change for another time.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>