services: dhcpd6 only run when prefix delegation is needed

src/etc/inc/services.inc

@@ -1054,9 +1054,10 @@ function is_dhcpv6_server_enabled()
 {
     global $config;
 
-    if (is_array($config['interfaces'])) {
-        foreach (legacy_config_get_interfaces(array("virtual" => false)) as $ifcfg) {
-            if (isset($ifcfg['enable']) && !empty($ifcfg['track6-interface'])) {
+    foreach (legacy_config_get_interfaces(array('virtual' => false)) as $ifcfg) {
+        if (isset($ifcfg['enable']) && !empty($ifcfg['track6-interface'])) {
+            $pdlen = calculate_ipv6_delegation_length($ifcfg['track6-interface']);
+            if ($pdlen > 2) {
                 return true;
             }
         }