cifs: wire up >migrate_folio

CIFS does not use page private data that needs migration, so it can just
wire up filemap_migrate_folio.  This prepares for removing ->writepage,
which is used as a fallback if no migrate_folio method is set.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>