ceph: fix null pointer dereference in send_mds_reconnect()

sb->s_root can be null when umounting
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/mds_client.c

@@ -2892,6 +2892,7 @@ static void send_mds_reconnect(struct ceph_mds_client *mdsc,
 	spin_unlock(&session->s_cap_lock);
 
 	/* trim unused caps to reduce MDS's cache rejoin time */
+	if (mdsc->fsc->sb->s_root)
 		shrink_dcache_parent(mdsc->fsc->sb->s_root);
 
 	ceph_con_close(&session->s_con);