The zone->pressure field is supposed to record the amount of reclaim pressure
which this zone is under.  We need this info so we know whether to unmap
pages from pagetables right from the outset of a balance_pgdat() or
try_to_free_pages() invokation.

The problem with the current code is that the exponential average gets tugged
around too much: as we perform the increasing-priority scan, the pressure
metric is made artificially low by the early part of the scan.

So instead what we do here is to record within the zone the scanning priority
from the zone's previous scan.  It is defined as the priority at which the
zone achieved the "enough pages free" state.  This prev_priority is used on
the next scan for the do-we-need-to-be-unmapping-pages decision.