kernel - SWAP CACHE part 22/many - Fix counter overflow introduced in part 21

kernel - SWAP CACHE part 22/many - Fix counter overflow introduced in part 21

* vm_swapcache_inactive_heuristic can overflow, making it extremely negative
  and causing swapcache writing to cease for a long period of time.

* Two-in-one fix.  Check if it is too negative and reset it.  This also
  automatically adjusts the heuristic if the sysop manually changes
  the hysteresis to a smaller value.