netfilter: netns ct: walk netns list under RTNL

netns list (just list) is under RTNL. But helper and proto unregistration
happen during rmmod when RTNL is not held, and that's how it was tested:
modprobe/rmmod vs clone(CLONE_NEWNET)/exit.

BUG: unable to handle kernel paging request at 0000000000100100	<===
IP: [<ffffffffa009890f>] nf_conntrack_l4proto_unregister+0x96/0xae [nf_conntrack]
PGD 15e300067 PUD 15e1d8067 PMD 0
Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
last sysfs file: /sys/kernel/uevent_seqnum
CPU 0
Modules linked in: nf_conntrack_proto_sctp(-) nf_conntrack_proto_dccp(-) af_packet iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack nf_defrag_ipv4 iptable_filter ip_tables xt_tcpudp ip6table_filter ip6_tables x_tables ipv6 sr_mod cdrom [last unloaded: nf_conntrack_proto_sctp]
Pid: 16758, comm: rmmod Not tainted 2.6.28-rc2-netns-xfrm #3
RIP: 0010:[<ffffffffa009890f>]  [<ffffffffa009890f>] nf_conntrack_l4proto_unregister+0x96/0xae [nf_conntrack]
RSP: 0018:ffff88015dc1fec8  EFLAGS: 00010212
RAX: 000...