SUNRPC: Allow caller to specify the transport to use

This is needed in order to allow the NFSv4.1 backchannel and
BIND_CONN_TO_SESSION function to work.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

include/linux/sunrpc/sched.h

@@ -103,6 +103,7 @@ struct rpc_call_ops {
 struct rpc_task_setup {
 	struct rpc_task *task;
 	struct rpc_clnt *rpc_client;
+	struct rpc_xprt *rpc_xprt;
 	const struct rpc_message *rpc_message;
 	const struct rpc_call_ops *callback_ops;
 	void *callback_data;

net/sunrpc/sched.c

@@ -909,6 +909,8 @@ static void rpc_init_task(struct rpc_task *task, const struct rpc_task_setup *ta
 	/* Initialize workqueue for async tasks */
 	task->tk_workqueue = task_setup_data->workqueue;
 
+	task->tk_xprt = xprt_get(task_setup_data->rpc_xprt);
+
 	if (task->tk_ops->rpc_call_prepare != NULL)
 		task->tk_action = rpc_prepare_task;