net: Convert nf_ct_net_ops

These pernet_operations register and unregister sysctl.
Also, there is inet_frags_exit_net() called in exit method,
which has to be safe after a560002437d3 "net: Fix hlist
corruptions in inet_evict_bucket()".

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -646,6 +646,7 @@ static void nf_ct_net_exit(struct net *net)
 static struct pernet_operations nf_ct_net_ops = {
 	.init = nf_ct_net_init,
 	.exit = nf_ct_net_exit,
+	.async = true,
 };
 
 int nf_ct_frag6_init(void)