kernel - Improve pageout daemon for tmpfs write load

kernel - Improve pageout daemon for tmpfs write load

* Rewrite vm_object_page_collect() and vm_pageout_clean() to align the
  set of pages being flushed to a swap cache stripe (~16-32 pages).

* Do a better job tracking how many pages have been cleaned by the pageout
  daemon.

* Setup an iterator and allow the inactive scan and the active scan to
  break out of their respective loops if they meet their quota early.

* Disable the swap iterator for now, it has not been shown to improve
  matters (yet).