From: Nikita Danilov <Nikita@Namesys.COM>

Use zone->pressure (rathar than scanning priority) to determine when to
start reclaiming mapped pages in refill_inactive_zone().  When using
priority every call to try_to_free_pages() starts with scanning parts of
active list and skipping mapped pages (because reclaim_mapped evaluates to
0 on low priorities) no matter how high memory pressure is.