mm: adjust page migration heuristic

The page allocator's heuristic to decide when to migrate page blocks to
unmovable seems to have been tuned on architectures that do not have
kernel drivers that would make unmovable allocations of several
megabytes or greater--ie, no cameras or shared-memory GPUs. The number
of allocations from these drivers may be unbounded and may occupy a
significant percentage of overall system memory (>50%). As a result,
every Android device has suffered to some extent from increasing
fragmentation due to unmovable page block migration over time.

This change adjusts the page migration heuristic to only migrate page
blocks for unmovable allocations when the order of the requested
allocation is order-5 or greater. This prevents migration due to GPU and
ion allocations so long as kernel drivers allocate memory at runtime
using order-4 or smaller pages.

Experimental results running the Android longevity test suite on a Nexus
5X for 10 hours:

old heuristic: 116 unmovable blocks after boot -> 281 unmovable blocks
new heuristic: 105 unmovable blocks after boot -> 101 unmovable blocks

bug 26916944

Change-Id: I5b7ccbbafa4049a2f47f399df4cb4779689f4c40
Signed-off-by: Alex Naidis <alex.naidis@linux.com>