selftests: netfilter: Fix missing return values in conntrack_dump_flush

Fix the bug of some functions were missing return values.

Fixes: eff3c55 ("netfilter: ctnetlink: support filtering by zone")
Signed-off-by: Guan Jing <guanjing@cmss.chinamobile.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: guanjing

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

@@ -43,6 +43,8 @@ static int build_cta_tuple_v4(struct nlmsghdr *nlh, int type,
 	mnl_attr_nest_end(nlh, nest_proto);
 
 	mnl_attr_nest_end(nlh, nest);
+
+	return 0;
 }
 
 static int build_cta_tuple_v6(struct nlmsghdr *nlh, int type,
@@ -71,6 +73,8 @@ static int build_cta_tuple_v6(struct nlmsghdr *nlh, int type,
 	mnl_attr_nest_end(nlh, nest_proto);
 
 	mnl_attr_nest_end(nlh, nest);
+
+	return 0;
 }
 
 static int build_cta_proto(struct nlmsghdr *nlh)
@@ -90,6 +94,8 @@ static int build_cta_proto(struct nlmsghdr *nlh)
 	mnl_attr_nest_end(nlh, nest_proto);
 
 	mnl_attr_nest_end(nlh, nest);
+
+	return 0;
 }
 
 static int conntrack_data_insert(struct mnl_socket *sock, struct nlmsghdr *nlh,