netfilter: conntrack: add nf_{tcp, udp, sctp, icmp, dccp, icmpv6, generic}_pernet()

mainline inclusion
from mainline-4.20
commit a95a7774
category: bugfix
bugzilla: 6008
CVE: NA

-------------------------------------------------

Expose these functions to access conntrack protocol tracker netns area,
nfnetlink_cttimeout needs this.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>

Conflicts:
  net/netfilter/nf_conntrack_proto_dccp.c
  net/netfilter/nf_conntrack_proto_generic.c
  net/netfilter/nf_conntrack_proto_icmp.c
  net/netfilter/nf_conntrack_proto_icmpv6.c
  net/netfilter/nf_conntrack_proto_sctp.c
  net/netfilter/nf_conntrack_proto_tcp.c
  net/netfilter/nf_conntrack_proto_udp.c
Reviewed-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>