pin_user_pages supports NULL pages arguments?

[Minchan Kim]

 * pin_user_pages() - pin user pages in memory for use by other devices
 < snip >
 * @pages:      array that receives pointers to the pages pinned.
 *              Should be at least nr_pages long. Or NULL, if caller
 *              only intends to ensure the pages are faulted in.

pin_user_pages(,, pages = NULL, );
  gup_flags |= FOLL_PIN
  __get_user_pages_locked
    __get_user_pages
      ..
      VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));