netfilter: xt_connlimit: honor conntrack zone if available

Currently all the conntrack lookups are done using default zone.
In case the skb has a ct attached (e.g. template) we should use this zone
for lookups instead.  This makes connlimit work with connections assigned
to other zones.

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