mm: numa: Migrate on reference policy

This is the simplest possible policy that still does something of note.
When a pte_numa is faulted, it is moved immediately. Any replacement
policy must at least do better than this and in all likelihood this
policy regresses normal workloads.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Rik van Riel <riel@redhat.com>