Another bug is that if try_to_free_pages / balance_pgdat are called
with a gfp_mask specifying GFP_IO and/or GFP_FS, they may reclaim
the requisite number of pages, and reset prev_priority to DEF_PRIORITY.

However, another reclaimer without those gfp_mask flags set may still
be struggling to reclaim pages. The easy fix for this is to key the
distress calculation not off zone->prev_priority, but also take into
account the local caller's priority by using:
min(zone->prev_priority, sc->priority)

Signed-off-by: Martin J. Bligh <mbligh@google.com>