netfilter: netns ebtables: part 1

* propagate netns from userspace, register table in passed netns
* remporarily register every ebt_table in init_net

P. S.: one needs to add ".netns_ok = 1" to igmp_protocol to test with
ebtables(8) in netns.

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