Style changes in vm_pageout_scan():

1. Be consistent in the style of "act_delta" manipulations between the
   inactive and active queue scans.

2. Explicitly compare to zero.

3. The deactivation of a page is based is based on its recent history
   and not just the current call to vm_pageout_scan().  The variable
   "act_delta" represents the current state of the page, and not its
   history.  Avoid possible confusion by not (ab)using "act_delta" for
   the making the deactivation decision.

Submitted by:	kib [1]
Reviewed by:	kib [2,3]