Remedy the following three problems:

1. The amd64 pmap, unlike the i386 pmap, maintains a reference count
   for each page directory (PD) page.  However, in the transformation
   of the i386 pmap into the amd64 pmap, operations, such as
   pmap_copy() and pmap_object_init_pt(), that create 2MB "superpage"
   mappings by setting the PG_PS bit in a PD entry were not modified
   to adjust the underlying PD page's reference count.  Consequently,
   superpage mappings could disappear prematurely.

2. pmap_object_init_pt() could crash or corrupt memory if either the
   virtual address range being mapped crosses a 1GB boundary in the
   virtual address space or nothing is mapped in the 1GB area.

3. When pmap_allocpte() destroys a 2MB "superpage" mapping it does not
   reduce the pmap's resident count accordingly.  It should.  (This
   bug is inherited from i386.)

Discussed with: peter
Reviewed by:    tegge

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