xen-netfront: always set num queues if possible

If netfront connects with two (or more) queues and then reconnects with
only one queue it fails to delete or rewrite the multi-queue-num-queues
key and netback will try to use the wrong number of queues.

Always write the num-queues field if the backend has multi-queue support.
Signed-off-by: NChas Williams <3chas3@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -1819,19 +1819,22 @@ static int talk_to_netback(struct xenbus_device *dev,
 		goto destroy_ring;
 	}
 
-	if (num_queues == 1) {
-		err = write_queue_xenstore_keys(&info->queues[0], &xbt, 0); /* flat */
-		if (err)
-			goto abort_transaction_no_dev_fatal;
-	} else {
+	if (xenbus_exists(XBT_NIL,
+			  info->xbdev->otherend, "multi-queue-max-queues")) {
 		/* Write the number of queues */
-		err = xenbus_printf(xbt, dev->nodename, "multi-queue-num-queues",
-				    "%u", num_queues);
+		err = xenbus_printf(xbt, dev->nodename,
+				    "multi-queue-num-queues", "%u", num_queues);
 		if (err) {
 			message = "writing multi-queue-num-queues";
 			goto abort_transaction_no_dev_fatal;
 		}
+	}
 
+	if (num_queues == 1) {
+		err = write_queue_xenstore_keys(&info->queues[0], &xbt, 0); /* flat */
+		if (err)
+			goto abort_transaction_no_dev_fatal;
+	} else {
 		/* Write the keys for each queue */
 		for (i = 0; i < num_queues; ++i) {
 			queue = &info->queues[i];