link: Check address families on a link always match

We can't create a link with a source address of a different
address family than the destination as all sends will fail.

libknet/links.c

@@ -146,6 +146,13 @@ int knet_link_set_config(knet_handle_t knet_h, knet_node_id_t host_id, uint8_t l
 		goto exit_unlock;
 	}
 
+	if (dst_addr && (src_addr->ss_family != dst_addr->ss_family)) {
+		log_err(knet_h, KNET_SUB_LINK, "Source address family does not match destination address family");
+		err = -1;
+		savederrno = EINVAL;
+		goto exit_unlock;
+	}
+
 	host = knet_h->host_index[host_id];
 	if (!host) {
 		err = -1;

libknet/tests/api_knet_link_set_config.c

@@ -113,6 +113,24 @@ static void test(void)
 
 	flush_logs(logfds[0], stdout);
 
+	printf("Test knet_link_set_config with conflicting address families\n");
+
+	if (make_local_sockaddr6(&dst, 1) < 0) {
+		printf("Unable to convert dst to sockaddr: %s\n", strerror(errno));
+		exit(FAIL);
+	}
+
+	if (knet_link_set_config(knet_h, 1, 0, KNET_TRANSPORT_UDP, &src, &dst, 0) == 0) {
+		printf("knet_link_set_config accepted invalid address families: %s\n", strerror(errno));
+		knet_host_remove(knet_h, 1);
+		knet_handle_free(knet_h);
+		flush_logs(logfds[0], stdout);
+		close_logpipes(logfds);
+		exit(FAIL);
+	}
+
+	flush_logs(logfds[0], stdout);
+
 	printf("Test knet_link_set_config with dynamic dst_addr\n");
 
 	if (knet_link_set_config(knet_h, 1, 0, KNET_TRANSPORT_UDP, &src, NULL, 0) < 0) {