netfilter: netns: remove nf_*_net() wrappers

Now that dev_net() exists, the usefullness of them is even less. Also they're
a big problem in resolving circular header dependencies necessary for
NOTRACK-in-netns patch. See below.

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