netfilter: move zone info into struct nf_conn

Curently we store zone information as a conntrack extension.
This has one drawback: for every lookup we need to fetch the zone data
from the extension area.

This change place the zone data directly into the main conntrack object
structure and then removes the zone conntrack extension.

The zone data is just 4 bytes, it fits into a padding hole before
the tuplehash info, so we do not even increase the nf_conn structure size.

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