netfilter: ctnetlink: don't add null bindings if no nat requested

commit 0eba801b tried to fix a race
where nat initialisation can happen after ctnetlink-created conntrack
has been created.

However, it causes the nat module(s) to be loaded needlessly on
systems that are not using NAT.

Fortunately, we do not have to create null bindings in that case.

conntracks injected via ctnetlink always have the CONFIRMED bit set,
which prevents addition of the nat extension in nf_nat_ipv4/6_fn().

We only need to make sure that either no nat extension is added
or that we've created both src and dst manips.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>