netfilter: enable netfilter in netns

From kernel perspective, allow entrance in nf_hook_slow().

Stuff which uses nf_register_hook/nf_register_hooks, but otherwise not netns-ready:

	DECnet netfilter
	ipt_CLUSTERIP
	nf_nat_standalone.c together with XFRM (?)
	IPVS
	several individual match modules (like hashlimit)
	ctnetlink
	NOTRACK
	all sorts of queueing and reporting to userspace
	L3 and L4 protocol sysctls, bridge sysctls
	probably something else

Anyway critical mass has been achieved, there is no reason to hide netfilter any longer.

From userspace perspective, allow to manipulate all sorts of
iptables/ip6tables/arptables rules.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>