[Finder] fix reverse sorting custom-ordering functions

Author: nicolas-grekas

src/Symfony/Component/Finder/Iterator/SortableIterator.php

@@ -73,7 +73,7 @@ public function __construct(\Traversable $iterator, $sort, bool $reverseOrder =
         } elseif (self::SORT_BY_NONE === $sort) {
             $this->sort = $order;
         } elseif (\is_callable($sort)) {
-            $this->sort = $sort;
+            $this->sort = $reverseOrder ? function ($a, $b) use ($sort) { return -\call_user_func($sort, $a, $b); } : $sort;
         } else {
             throw new \InvalidArgumentException('The SortableIterator takes a PHP callable or a valid built-in sort algorithm as an argument.');
         }