net: Convert ctnetlink_net_ops

These pernet_operations register and unregister
two conntrack notifiers, and they seem to be safe
to be executed in parallel.

General/not related to async pernet_operations JFI:
ctnetlink_net_exit_batch() actions are grouped in batch,
and this could look like there is synchronize_rcu()
is forgotten. But there is synchronize_rcu() on module
exit patch (in ctnetlink_exit()), so this batch may
be reworked as simple .exit method.

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