Three enhancements to arm64's pmap_protect():

Implement protection changes on superpage mappings. Previously, a superpage
mapping was unconditionally demoted by pmap_protect(), even if the
protection change applied to the entire superpage mapping.

Precompute the bit mask describing the protection changes rather than
recomputing it for every page table entry that is changed.

Skip page table entries that already have the requested protection changes
in place.

Reviewed by: andrew, kib
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D20657