SUNRPC: Add the helper function 'rpc_call_null()'

Does a NULL RPC call and returns a pointer to the resulting rpc_task. The
call may be either synchronous or asynchronous.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/clnt.h

@@ -130,6 +130,8 @@ int		rpc_call_async(struct rpc_clnt *clnt, struct rpc_message *msg,
 			       void *calldata);
 int		rpc_call_sync(struct rpc_clnt *clnt, struct rpc_message *msg,
 			      int flags);
+struct rpc_task *rpc_call_null(struct rpc_clnt *clnt, struct rpc_cred *cred,
+			       int flags);
 void		rpc_restart_call(struct rpc_task *);
 void		rpc_clnt_sigmask(struct rpc_clnt *clnt, sigset_t *oldset);
 void		rpc_clnt_sigunmask(struct rpc_clnt *clnt, sigset_t *oldset);

net/sunrpc/clnt.c

@@ -1455,6 +1455,16 @@ static int rpc_ping(struct rpc_clnt *clnt, int flags)
 	return err;
 }
 
+struct rpc_task *rpc_call_null(struct rpc_clnt *clnt, struct rpc_cred *cred, int flags)
+{
+	struct rpc_message msg = {
+		.rpc_proc = &rpcproc_null,
+		.rpc_cred = cred,
+	};
+	return rpc_do_run_task(clnt, &msg, flags, &rpc_default_ops, NULL);
+}
+EXPORT_SYMBOL(rpc_call_null);
+
 #ifdef RPC_DEBUG
 void rpc_show_tasks(void)
 {