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>
diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index d32f49d..cb002dc 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -589,6 +589,12 @@
 	do_probe_callback(clp);
 }
 
+void nfsd4_probe_callback_sync(struct nfs4_client *clp)
+{
+	nfsd4_probe_callback(clp);
+	flush_workqueue(callback_wq);
+}
+
 void nfsd4_change_callback(struct nfs4_client *clp, struct nfs4_cb_conn *conn)
 {
 	clp->cl_cb_state = NFSD4_CB_UNKNOWN;