xen-netback: stop the guest rx thread after a fatal error

After commit e9d8b2c2968499c1f96563e6522c56958d5a1d0d (xen-netback:
disable rogue vif in kthread context), a fatal (protocol) error would
leave the guest Rx thread spinning, wasting CPU time. Commit
ecf08d2dbb96d5a4b4bcc53a39e8d29cc8fef02e (xen-netback: reintroduce
guest Rx stall detection) made this even worse by removing a
cond_resched() from this path.

Since a fatal error is non-recoverable, just allow the guest Rx thread
to exit. This requires taking additional refs to the task so the
thread exiting early is handled safely.

Signed-off-by: David Vrabel <[e-mail]>
Reported-by: Julien Grall <[e-mail]>
Tested-by: Julien Grall <[e-mail]>
Acked-by: Wei Liu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/xen-netback/interface.c

···
 		goto err_rx_unbind;
 	}
 	queue->task = task;
+	get_task_struct(task);
 
 	task = kthread_create(xenvif_dealloc_kthread,
 			      (void *)queue, "%s-dealloc", queue->name);
···
 
 		if (queue->task) {
 			kthread_stop(queue->task);
+			put_task_struct(queue->task);
 			queue->task = NULL;
 		}
 

drivers/net/xen-netback/netback.c

···
 		 */
 		if (unlikely(vif->disabled && queue->id == 0)) {
 			xenvif_carrier_off(vif);
-			xenvif_rx_queue_purge(queue);
-			continue;
+			break;
 		}
 
 		if (!skb_queue_empty(&queue->rx_queue))