amd64: Zero the PML5 PTI page when initializing a pmap

The root page is not zeroed at allocation time since with 4-level tables
each entry is copied from a template. However, with 5-level tables only
a single entry is filled, so the rest need to be cleared.

Reported by: alc
Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b0423d0f5eec89bd6c11c32649603031e3dbbfe1)