xen-netfront: fix oops when disconnected from backend

xennet_disconnect_backend() was not correctly iterating over all the
queues.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -1437,10 +1437,11 @@ static void xennet_end_access(int ref, void *page)
 static void xennet_disconnect_backend(struct netfront_info *info)
 {
 	unsigned int i = 0;
-	struct netfront_queue *queue = NULL;
 	unsigned int num_queues = info->netdev->real_num_tx_queues;
 
 	for (i = 0; i < num_queues; ++i) {
+		struct netfront_queue *queue = &info->queues[i];
+
 		/* Stop old i/f to prevent errors whilst we rebuild the state. */
 		spin_lock_bh(&queue->rx_lock);
 		spin_lock_irq(&queue->tx_lock);