A shmem cleanup/bugfix patch from Hugh Dickins.

- Minor: in try_to_unuse(), only wait on writeout if we actually
  started new writeout.  Otherwise, there is no need because a
  wait_on_page_writeback() has already been executed against this page.
  And it's locked, so no new writeback can start.

- Minor: in shmem_unuse_inode(): remove all the
  wait_on_page_writeback() logic.  We already did that in
  try_to_unuse(), adn the page is locked so no new writeback can start.

- Less minor: add a missing a page_cache_release() to
  shmem_get_page_locked() in the uncommon case where the page was found
  to be under writeout.