SUNRPC: Add a per-rpc_clnt spinlock

Use that to protect the rpc_clnt->cl_tasks list instead of using a global
lock.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/clnt.h

@@ -28,6 +28,7 @@ struct rpc_clnt {
 	atomic_t		cl_users;	/* number of references */
 	struct list_head	cl_clients;	/* Global list of clients */
 	struct list_head	cl_tasks;	/* List of tasks */
+	spinlock_t		cl_lock;	/* spinlock */
 	struct rpc_xprt *	cl_xprt;	/* transport */
 	struct rpc_procinfo *	cl_procinfo;	/* procedure info */
 	u32			cl_prog,	/* RPC program number */