net: Convert arp_tables_net_ops and ip6_tables_net_ops

These pernet_operations call xt_proto_init() and xt_proto_fini(),
which just register and unregister /proc entries.
They are safe to be marked as async.

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