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>

fs/ceph/mds_client.c

@@ -3685,6 +3685,12 @@ static void handle_session(struct ceph_mds_session *session,
 		break;
 
 	case CEPH_SESSION_FLUSHMSG:
+		/* flush cap releases */
+		spin_lock(&session->s_cap_lock);
+		if (session->s_num_cap_releases)
+			ceph_flush_cap_releases(mdsc, session);
+		spin_unlock(&session->s_cap_lock);
+
 		send_flushmsg_ack(mdsc, session, seq);
 		break;