CIFS: Add launder_page operation (try #3)

Add this let us drop filemap_write_and_wait from cifs_invalidate_mapping
and simplify the code to properly process invalidate logic.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>