From: Nikita Danilov <Nikita@Namesys.COM>

The vmscan logic at present will scan the inactive list with increasing
priority until a threshold is triggered.  At that threshold we start
unmapping pages from pagetables.

The problem is that each time someone calls into this code, the priority is
initially low, so some mapped pages will be refiled event hough we really
should be unmapping them now.

Nikita's patch adds the `pressure' field to struct zone.  it is a decaying
average of the zone's memory pressure and allows us to start unmapping pages
immediately on entry to page reclaim, based on measurements which were made
in earlier reclaim attempts.