net: Convert lowpan_frags_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 a5600024 "net: Fix hlist
corruptions in inet_evict_bucket()".
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan/reassembly.c

@@ -603,6 +603,7 @@ static void __net_exit lowpan_frags_exit_net(struct net *net)
 static struct pernet_operations lowpan_frags_ops = {
 	.init = lowpan_frags_init_net,
 	.exit = lowpan_frags_exit_net,
+	.async = true,
 };
 
 int __init lowpan_net_frag_init(void)