kernel - Fix swapcache inefficiency related to the PQ_INACTIVE queue

kernel - Fix swapcache inefficiency related to the PQ_INACTIVE queue

* The swapcache was still coded with the assumption of just one
  PQ_INACTIVE queue.  Even though it scans all 256 queues the
  end-of-queue code could trigger early and prevent later queue
  indices from being scanned, causing a large percentage of eligible
  VM pages to not get written to the swapcache.

  Now all queues must signal that they have reached the end before the
  code sets the end-of-queue condition and loops up to the 1/10 second
  sleep.

* The full count/scount was being applied to each queue individually instead
  of to all queues uniformly, allowing more pages to be laundered than
  desired.  Divide out and add a bit of slop.