selftests: netfilter: Add missing return value

There is no return value in count_entries, just add it.

Fixes: eff3c558 ("netfilter: ctnetlink: support filtering by zone")
Signed-off-by: zhang jiao <zhangjiao2@cmss.chinamobile.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>