MFC r203085   Optimize pmap_demote_pde() by using the new KPTmap to access a kernel   page table page instead of creating a temporary mapping to it.
MFC r203085
  Optimize pmap_demote_pde() by using the new KPTmap to access a kernel
  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().

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