packet: exit_net cleanup check added

Be sure that packet.sklist initialized in net_init hook was return
to initial state.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -4560,6 +4560,7 @@ static int __net_init packet_net_init(struct net *net)
 static void __net_exit packet_net_exit(struct net *net)
 {
 	remove_proc_entry("packet", net->proc_net);
+	WARN_ON_ONCE(!hlist_empty(&net->packet.sklist));
 }
 
 static struct pernet_operations packet_net_ops = {