Remove PG_U from the rest of the kernel pmap ptes.

Supposedly, they PG_U bits there were set to easier making some kernel
page accessible to userspace in-place. Since it was not used for the
whole existence of the amd64 pmap.c and current design of the shared
pages prefers double-mapping over the in-place access, remove PG_U
both from the direct map and KVA slots.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week