fs: fix ReferenceError in truncateSync()

Fixes a minor oversight introduced in 168a555, resulting in the following error:

  fs.js:467
      return fs.ftruncateSync(path, len, callback);
                                         ^
  ReferenceError: callback is not defined
      at Object.fs.truncateSync (fs.js:467:40)

lib/fs.js

@@ -464,7 +464,7 @@ fs.truncate = function(path, len, callback) {
 fs.truncateSync = function(path, len) {
   if (typeof path === 'number') {
     // legacy
-    return fs.ftruncateSync(path, len, callback);
+    return fs.ftruncateSync(path, len);
   }
   if (typeof len === 'undefined') {
     len = 0;