netfilter: nft_ct: add zone id set support

zones allow tracking multiple connections sharing identical tuples,
this is needed e.g. when tracking distinct vlans with overlapping ip
addresses (conntrack is l2 agnostic).

Thus the zone has to be set before the packet is picked up by the
connection tracker.  This is done by means of 'conntrack templates' which
are conntrack structures used solely to pass this info from one netfilter
hook to the next.

The iptables CT target instantiates these connection tracking templates
once per rule, i.e. the template is fixed/tied to particular zone, can
be read-only and therefore be re-used by as many skbs simultaneously as
needed.

We can't follow this model because we want to take the zone id from
an sreg at rule eval time so we could e.g. fill in the zone id from
the packets vlan id or a e.g. nftables key : value maps.

To avoid cost of per packet alloc/free of the template, use a percpu
template 'scratch' object and use the refcount to detect the (unlikely)
case where the template is still attached to another skb (i.e., previous
skb was nfqueued ...).

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