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
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32525