ceph: redirty page before returning AOP_WRITEPAGE_ACTIVATE

The page has been marked clean before writepage is called.  If we don't
redirty it before postponing the write, it might never get written.

Cc: stable@vger.kernel.org
Fixes: 503d4fa6 ("ceph: remove reliance on bdi congestion")
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>