netfilter: ip6table_filter in netns for real

One still needs to remove checks in nf_hook_slow() and nf_sockopt_find()
to test this, though.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>