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>

tools/testing/selftests/net/netfilter/conntrack_dump_flush.c

@@ -207,6 +207,7 @@ static int conntrack_data_generate_v6(struct mnl_socket *sock,
 static int count_entries(const struct nlmsghdr *nlh, void *data)
 {
 	reply_counter++;
+	return MNL_CB_OK;
 }
 
 static int conntracK_count_zone(struct mnl_socket *sock, uint16_t zone)