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>
diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 1b74420..aa0449d 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -181,6 +181,7 @@
 	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 @@
 		__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));
 }