SUNRPC: track length of RPC wait queues

RPC wait queue length will eventually be exported to userland via the RPC
iostats interface.

Test plan:
Compile kernel with CONFIG_NFS enabled.
Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/sched.h

@@ -203,6 +203,7 @@ struct rpc_wait_queue {
 	unsigned char		priority;		/* current priority */
 	unsigned char		count;			/* # task groups remaining serviced so far */
 	unsigned char		nr;			/* # tasks remaining for cookie */
+	unsigned short		qlen;			/* total # tasks waiting in queue */
 #ifdef RPC_DEBUG
 	const char *		name;
 #endif

net/sunrpc/sched.c

@@ -181,6 +181,7 @@ static void __rpc_add_wait_queue(struct rpc_wait_queue *queue, struct rpc_task *
 	else
 		list_add_tail(&task->u.tk_wait.list, &queue->tasks[0]);
 	task->u.tk_wait.rpc_waitq = queue;
+	queue->qlen++;
 	rpc_set_queued(task);
 
 	dprintk("RPC: %4d added to queue %p \"%s\"\n",
@@ -215,6 +216,7 @@ static void __rpc_remove_wait_queue(struct rpc_task *task)
 		__rpc_remove_wait_queue_priority(task);
 	else
 		list_del(&task->u.tk_wait.list);
+	queue->qlen--;
 	dprintk("RPC: %4d removed from queue %p \"%s\"\n",
 				task->tk_pid, queue, rpc_qname(queue));
 }