ceph: flush cap releases when the session is flushed

MDS expects the completed cap release prior to responding to the
session flush for cache drop.

Cc: stable@vger.kernel.org
Link: http://tracker.ceph.com/issues/38009
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>