MFC r314310

MFC r314310
  Refine the fix from r312954.  Specifically, add a new PDE-only flag,
  PG_PROMOTED, that indicates whether lingering 4KB page mappings might
  need to be flushed on a PDE change that restricts or destroys a 2MB
  page mapping.  This flag allows the pmap to avoid range invalidations
  that are both unnecessary and costly.

Approved by:	re (kib)