When we protect PTEs (as opposed to PDEs), we only call vm_page_dirty()
when, in fact, we are write protecting the page and the PTE has PG_M set.
However, pmap_protect_pde() was always calling vm_page_dirty() when the PDE
has PG_M set. So, adding PG_NX to a writeable PDE could result in
unnecessary (but harmless) calls to vm_page_dirty().

Simplify the loop calling vm_page_dirty() in pmap_protect_pde().

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20793