ipv6: speedup ipv6 tunnels dismantle

Implement exit_batch() method to dismantle more devices
per round.

(rtnl_lock() ...
 unregister_netdevice_many() ...
 rtnl_unlock())

Tested:
$ cat add_del_unshare.sh
for i in `seq 1 40`
do
 (for j in `seq 1 100` ; do unshare -n /bin/true >/dev/null ; done) &
done
wait ; grep net_namespace /proc/slabinfo

Before patch :
$ time ./add_del_unshare.sh
net_namespace        110    267   5504    1    2 : tunables    8    4    0 : slabdata    110    267      0

real    3m25.292s
user    0m0.644s
sys     0m40.153s

After patch:

$ time ./add_del_unshare.sh
net_namespace        126    282   5504    1    2 : tunables    8    4    0 : slabdata    126    282      0

real	1m38.965s
user	0m0.688s
sys	0m37.017s

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -1155,19 +1155,21 @@ err_alloc_dev:
 	return err;
 }
 
-static void __net_exit ip6gre_exit_net(struct net *net)
+static void __net_exit ip6gre_exit_batch_net(struct list_head *net_list)
 {
+	struct net *net;
 	LIST_HEAD(list);
 
 	rtnl_lock();
-	ip6gre_destroy_tunnels(net, &list);
+	list_for_each_entry(net, net_list, exit_list)
+		ip6gre_destroy_tunnels(net, &list);
 	unregister_netdevice_many(&list);
 	rtnl_unlock();
 }
 
 static struct pernet_operations ip6gre_net_ops = {
 	.init = ip6gre_init_net,
-	.exit = ip6gre_exit_net,
+	.exit_batch = ip6gre_exit_batch_net,
 	.id   = &ip6gre_net_id,
 	.size = sizeof(struct ip6gre_net),
 };