selftests: netfilter: Fix missing return values in conntrack_dump_flush

Fix the bug of some functions were missing return values.

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