ctnetlink: compute generic part of event more acurately

On a box with most of the optional Netfilter switches turned off some
of the NLAs are never send, e. g. secmark, mark or the conntrack
byte/packet counters.  As a worst case scenario this may possibly
still lead to ctnetlink skbs being reallocated in netlink_trim()
later, loosing all the nice effects from the previous patches.

I try to solve that (at least partly) by correctly #ifdef'ing the
NLAs in the computation.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>