services: dhcpd6 only run when prefix delegation is needed

(cherry picked from commit 3336c43d)

src/etc/inc/services.inc

@@ -1057,10 +1057,13 @@ function is_dhcpv6_server_enabled()
     if (is_array($config['interfaces'])) {
         foreach ($config['interfaces'] as $ifcfg) {
             if (isset($ifcfg['enable']) && !empty($ifcfg['track6-interface'])) {
+                $pdlen = calculate_ipv6_delegation_length($ifcfg['track6-interface']);
+                if ($pdlen > 2) {
                     return true;
                 }
             }
         }
+    }
 
     if (!isset($config['dhcpdv6']) || !is_array($config['dhcpdv6'])) {
         return false;