Additional filter options were not passed to LDAP search in seachfordn().

This would mean that any code using this helper function could not
use the ldap.filter option. It's a followup to commit
ff5497e9 which fixed this for the
`login()` method but not for the `searchfordn()` method.

modules/ldap/lib/ConfigHelper.php

@@ -275,7 +275,7 @@ class sspmod_ldap_ConfigHelper
         }
 
         return $ldap->searchfordn($this->searchBase, $attribute,
-            $value, $allowZeroHits);
+            $value, $allowZeroHits, $this->searchFilter);
     }
 
     public function getAttributes($dn, $attributes = null)