net: Convert synproxy_net_ops

These pernet_operations create and destroy /proc entries
and allocate extents to template ct, which depend on global
nf_ct_ext_types[] array. So, we are able to mark them async.

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