SUNRPC: Fix a race in rpc_info_open

There is a race between rpc_info_open and rpc_release_client()
in that nothing stops a process from opening the file after
the clnt->cl_kref goes to zero.

Fix this by using atomic_inc_unless_zero()...

Reported-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org

include/linux/sunrpc/clnt.h

@@ -30,7 +30,7 @@ struct rpc_inode;
  * The high-level client handle
  */
 struct rpc_clnt {
-	struct kref		cl_kref;	/* Number of references */
+	atomic_t		cl_count;	/* 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 */