MFC r260567
  Correctly update the count of stuck pages, "addl_page_shortage", in
  vm_pageout_scan().  There were missing increments in two less common
  cases.

  Don't conflate the count of stuck pages and the pageout deficit provided
  by vm_page_alloc{,_contig}().

  Handle held pages consistently in the inactive queue scan.  In the more
  common case, we did not move the page to the tail of the queue.  Whereas,
  in the less common case, we did.  There's no particular reason to move
  the page in the less common case, so remove it.

  Perform the calculation of the page shortage for the active queue scan a
  little earlier, before the active queue lock is acquired.  The correctness
  of this calculation doesn't depend on the active queue lock being held.

  Eliminate a redundant variable, "pcount".  Use the more descriptive
  variable, "maxscan", in its place.

  Apply a few nearby style fixes, e.g., eliminate stray whitespace and
  excess parentheses.
MFC r265418
  Prior to r254304, a separate function, vm_pageout_page_stats(), was used
  to periodically update the reference status of the active pages.  This
  function was called, instead of vm_pageout_scan(), when memory was not
  scarce.  The objective was to provide up to date reference status for
  active pages in case memory did become scarce and active pages needed to
  be deactivated.

  The active page queue scan performed by vm_pageout_page_stats() was
  virtually identical to that performed by vm_pageout_scan(), and so r254304
  eliminated vm_pageout_page_stats().  Instead, vm_pageout_scan() is
  called with the parameter "pass" set to zero.  The intention was that when
  pass is zero, vm_pageout_scan() would only scan the active queue.
  However, the variable page_shortage can still be greater than zero when
  memory is not scarce and vm_pageout_scan() is called with pass equal to
  zero.  Consequently, the inactive queue may be scanned and dirty pages
  laundered even though that was not intended by r254304.  This revision
  fixes that.