xen-netfront: respect user provided max_queues

Originally that parameter was always reset to num_online_cpus during
module initialisation, which renders it useless.

The fix is to only set max_queues to num_online_cpus when user has not
provided a value.
Signed-off-by: NWei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: NDavid Vrabel <david.vrabel@citrix.com>
Tested-by: NDavid Vrabel <david.vrabel@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -2132,8 +2132,11 @@ static int __init netif_init(void)
 
 	pr_info("Initialising Xen virtual ethernet driver\n");
 
-	/* Allow as many queues as there are CPUs, by default */
-	xennet_max_queues = num_online_cpus();
+	/* Allow as many queues as there are CPUs if user has not
+	 * specified a value.
+	 */
+	if (xennet_max_queues == 0)
+		xennet_max_queues = num_online_cpus();
 
 	return xenbus_register_frontend(&netfront_driver);
 }