netfilter: nf_tables: keep chain counters away from hot path

These chain counters are only used by the iptables-compat tool, that
allow users to use the x_tables extensions from the existing nf_tables
framework. This patch makes nf_tables by ~5% for the general usecase,
ie. native nft users, where no chain counters are used at all.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>