page table page instead of creating a temporary mapping to it.
Set the PG_G bit on the page table entries that implement the KPTmap.
Locore initializes the unused portions of the NKPT kernel page table
pages that it allocates to zero. So, pmap_bootstrap() needn't zero
the page table entries referenced by CMAP1 and CMAP3.
Simplify pmap_set_pg().
MFC after: 10 days