Introduce pmap_change_prot() for amd64.

This updates the protection attributes of subranges of the kernel map.
Unlike pmap_protect(), which is typically used for user mappings,
pmap_change_prot() does not perform lazy upgrades of protections.
pmap_change_prot() also updates the aliasing range of the direct map.

Reviewed by: kib
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21758