swapper_space is different.  Its pages are locked during writeout, it
uses PAGE_SIZE rather than PAGE_CACHE_SIZE.  Converting swap to look
more like the other address_spaces is a separate project.  This patch
fully restores the old swap behaviour.

- Don't dirty swapcache page buffers in set_page_dirty().

  Fixes a problem where __free_pte() runs set_page_dirty() and then
  immediately runs ClearPageDirty().  The pages ends up clean, with
  dirty buffers, and is unfreeable.

- Hence, don't mark the page clean if its buffers are clean - swap
  does not have page/buffers dirty state coherency.