net: Convert ipv4_net_ops

These pernet_operations register and unregister bunch
of nf_conntrack_l4proto. Exit method unregisters related
sysctl, init method calls init_net and get_net_proto.
The whole builtin_l4proto4 array has pretty simple
init_net and get_net_proto methods. The first one register
sysctl table, the second one is just RO memory dereference.
So, these pernet_operations are safe to be marked as async.

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