nfsd4: make sure sequence flags are set after destroy_session

If this loses any backchannel, make sure we have a chance to notice that
and set the sequence flags.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1686,8 +1686,7 @@ nfsd4_destroy_session(struct svc_rqst *r,
 	spin_unlock(&client_lock);
 
 	nfs4_lock_state();
-	/* wait for callbacks */
-	nfsd4_shutdown_callback(ses->se_client);
+	nfsd4_probe_callback_sync(ses->se_client);
 	nfs4_unlock_state();
 
 	nfsd4_del_conns(ses);