NFSD: NFSv4 callback client should use RPC_TASK_SOFTCONN

The server's callback client should stop trying to connect to the
client's callback server as soon as it gets ECONNREFUSED.

The NFS server's callback client does not call rpc_ping(), but appears
to have it's own "ping" procedure, so it wasn't covered by commit
caabea8a.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4callback.c

@@ -542,7 +542,8 @@ void do_probe_callback(struct nfs4_client *clp)
 	};
 	int status;
 
-	status = rpc_call_async(cb->cb_client, &msg, RPC_TASK_SOFT,
+	status = rpc_call_async(cb->cb_client, &msg,
+				RPC_TASK_SOFT | RPC_TASK_SOFTCONN,
 				&nfsd4_cb_probe_ops, (void *)clp);
 	if (status) {
 		warn_no_callback_path(clp, status);