rpc_call_null_helper
return rpc_call_null_helper(clnt, NULL, cred, flags, NULL, NULL);
task = rpc_call_null_helper(clnt, NULL, NULL, 0, NULL, NULL);
task = rpc_call_null_helper(clnt, xprt, NULL, RPC_TASK_ASYNC,
task = rpc_call_null_helper(clnt, xprt, NULL, 0, NULL, NULL);