Pass stat buffer down through all levels of VFS_READDIR wrappers

Pass stat buffer down through all levels of VFS_READDIR wrappers

* VFS_OP_READDIR can now provide stat information, take advantage of it
  if it's available
* is_visible_file(): optimistically expect the provided stat buffer is
  already valid
* dptr_ReadDirName(): refactor code for easier readability, functionality
  is the same