interfaces.php, vlan mtu, check with actual parent

src/www/interfaces.php

@@ -814,14 +814,10 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
             }
 
             if (stristr($a_interfaces[$if]['if'], "_vlan")) {
-                $realhwif_array = get_parent_interface($a_interfaces[$if]['if']);
-                // Need code to handle MLPPP if we ever use $realhwif for MLPPP handling
-                $parent_realhwif = $realhwif_array[0];
-                $parent_if = convert_real_interface_to_friendly_interface_name($parent_realhwif);
-                if (!empty($parent_if) && !empty($config['interfaces'][$parent_if]['mtu'])) {
-                    if ($pconfig['mtu'] > intval($config['interfaces'][$parent_if]['mtu'])) {
-                        $input_errors[] = gettext("MTU of a vlan should not be bigger than parent interface.");
-                    }
+                $parentif = get_parent_interface($a_interfaces[$if]['if'])[0];
+                $intf_details = legacy_interfaces_details($parentif)[$parentif];
+                if ($intf_details['mtu'] < $pconfig['mtu']) {
+                    $input_errors[] = gettext("MTU of a vlan should not be bigger than parent interface.");
                 }
             } else {
                 foreach ($config['interfaces'] as $idx => $ifdata) {