netfilter: nf_tables: do not dump chain counters if not enabled

Chain counters are only enabled on demand since 9f08ea84, skip them
when dumping them via netlink.

Fixes: 9f08ea84 ("netfilter: nf_tables: keep chain counters away from hot path")
Reported-by: Johny Mattsson <johny.mattsson+kernel@gmail.com>
Tested-by: Johny Mattsson <johny.mattsson+kernel@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>