netfilter: netns nf_conntrack: add netns boilerplate

One comment: #ifdefs around #include is necessary to overcome amazing compile
breakages in NOTRACK-in-netns patch (see below).

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