- If grab_cache_page_nowait() is to be called while holding a lock on
  a different page, it must perform memory allocations with GFP_NOFS.
  Otherwise it could come back onto the locked page (if it's dirty) and
  deadlock.

  Also tidy this function up a bit - the checks in there were overly
  paranoid.

- In a few of places, look to see if we can avoid a buslocked cycle
  and dirtying of a cacheline.