netfilter: nf_conntrack: prepare namespace support for l3 protocol trackers

This patch prepares the namespace support for layer 3 protocol trackers.
Basically, this modifies the following interfaces:

* nf_ct_l3proto_[un]register_sysctl.
* nf_conntrack_l3proto_[un]register.

We add a new nf_ct_l3proto_net is used to get the pernet data of l3proto.

This adds rhe new struct nf_ip_net that is used to store the sysctl header
and l3proto_ipv4,l4proto_tcp(6),l4proto_udp(6),l4proto_icmp(v6) because the
protos such tcp and tcp6 use the same data,so making nf_ip_net as a field
of netns_ct is the easiest way to manager it.

This patch also adds init_net to struct nf_conntrack_l3proto to initial
the layer 3 protocol pernet data.

Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>