Avoid setting PG_U unconditionally in pmap_enter_quick_locked().

This KPI may in principle be used to create kernel mappings, in which
case we certainly should not be setting PG_U. In any case, PG_U must be
set on all layers in the page tables to grant user mode access, and we
were only setting it on leaf entries. Thus, this change should have no
functional impact.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation