netconsole: take care of NETDEV_UNREGISTER event

When netconsole is loaded and a network interface fades away (e.g. on
rmmod $interface_driver_module) the rmmod remains stuck and some locks
are taken that prevent any additional module loading/unloading as well
as interface up/down changes.
In addition kernel logs (and console) get flooded at 10s interval with

[  122.464065] unregister_netdevice: waiting for eth0 to become free. Usage count = 1
[  132.704059] unregister_netdevice: waiting for eth0 to become free. Usage count = 1

This patch lets netconsole take NETDEV_UNREGISTER event into account
and release the affected interface if it was in use.
Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netconsole.c

@@ -664,7 +664,7 @@ static int netconsole_netdev_event(struct notifier_block *this,
 	struct netconsole_target *nt;
 	struct net_device *dev = ptr;
 
-	if (!(event == NETDEV_CHANGENAME))
+	if (!(event == NETDEV_CHANGENAME || event == NETDEV_UNREGISTER))
 		goto done;
 
 	spin_lock_irqsave(&target_list_lock, flags);
@@ -675,6 +675,15 @@ static int netconsole_netdev_event(struct notifier_block *this,
 			case NETDEV_CHANGENAME:
 				strlcpy(nt->np.dev_name, dev->name, IFNAMSIZ);
 				break;
+			case NETDEV_UNREGISTER:
+				if (!nt->enabled)
+					break;
+				netpoll_cleanup(&nt->np);
+				nt->enabled = 0;
+				printk(KERN_INFO "netconsole: network logging stopped"
+					", interface %s unregistered\n",
+					dev->name);
+				break;
 			}
 		}
 		netconsole_target_put(nt);