handle everything from ctnetlink directly.

After all these years we still only support ipv4 and ipv6, so it
seems reasonable to remove l3 protocol tracker support and instead
handle ipv4/ipv6 from a common, always builtin inet tracker.

Step 1: Get rid of all the l3proto->func() calls.

Start with ctnetlink, then move on to packet-path ones.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>