net: Convert nf_conntrack_net_ops

These pernet_operations register and unregister sysctl and /proc
entries. Exit batch method also waits till all per-net conntracks
are dead. Thus, 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>