ceph: fix NULL pointer dereference in ceph_flush_snaps()

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -1469,7 +1469,7 @@ void ceph_flush_snaps(struct ceph_inode_info *ci,
 
 	if (psession) {
 		*psession = session;
-	} else {
+	} else if (session) {
 		mutex_unlock(&session->s_mutex);
 		ceph_put_mds_session(session);
 	}