SUNRPC: Capture completion of all RPC tasks

RPC tasks on the backchannel never invoke xprt_complete_rqst(), so
there is no way to report their tk_status at completion. Also, any
RPC task that exits via rpc_exit_task() before it is replied to will
also disappear without a trace.

Introduce a trace point that is symmetrical with rpc_task_begin that
captures the termination status of each RPC task.

Sample trace output for callback requests initiated on the server:
kworker/u8:12-448 [003] 127.025240: rpc_task_end: task:50@3 flags=ASYNC|DYNAMIC|SOFT|SOFTCONN|SENT runstate=RUNNING|ACTIVE status=0 action=rpc_exit_task
kworker/u8:12-448 [002] 127.567310: rpc_task_end: task:51@3 flags=ASYNC|DYNAMIC|SOFT|SOFTCONN|SENT runstate=RUNNING|ACTIVE status=0 action=rpc_exit_task
kworker/u8:12-448 [001] 130.506817: rpc_task_end: task:52@3 flags=ASYNC|DYNAMIC|SOFT|SOFTCONN|SENT runstate=RUNNING|ACTIVE status=0 action=rpc_exit_task

Odd, though, that I never see trace_rpc_task_complete, either in the
forward or backchannel. Should it be removed?

Signed-off-by: Chuck Lever <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

include/trace/events/sunrpc.h

···
 DEFINE_RPC_RUNNING_EVENT(begin);
 DEFINE_RPC_RUNNING_EVENT(run_action);
 DEFINE_RPC_RUNNING_EVENT(complete);
+DEFINE_RPC_RUNNING_EVENT(end);
 
 DECLARE_EVENT_CLASS(rpc_task_queued,
 

net/sunrpc/sched.c

···
  */
 void rpc_exit_task(struct rpc_task *task)
 {
+	trace_rpc_task_end(task, task->tk_action);
 	task->tk_action = NULL;
 	if (task->tk_ops->rpc_count_stats)
 		task->tk_ops->rpc_count_stats(task, task->tk_calldata);