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