[NET]: Check RTNL status in unregister_netdevice

The caller must hold the RTNL so let's check it in unregister_netdevice.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3971,6 +3971,8 @@ void synchronize_net(void)
 
 void unregister_netdevice(struct net_device *dev)
 {
+	ASSERT_RTNL();
+
 	rollback_registered(dev);
 	/* Finish processing unregister after unlock */
 	net_set_todo(dev);