setup: Convert octal directory mode to decimal notation

PHP's chmod sets wrong permissions when using 2770 as directory mode for example.

fixes #8233

modules/setup/library/Setup/Utils/MakeDirStep.php

@@ -16,12 +16,12 @@ class MakeDirStep extends Step
 
     /**
      * @param array $paths
-     * @param int   $dirmode
+     * @param int   $dirmode    Directory mode in octal notation
      */
     public function __construct($paths, $dirmode)
     {
         $this->paths = $paths;
-        $this->dirmode = $dirmode;
+        $this->dirmode = octdec($dirmode);
         $this->errors = array();
     }