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.

Reviewed by: kib, markj
MFC after: 6 weeks
Differential Revision: https://reviews.freebsd.org/D9665