Revision 187414

MFC rev 180362
  Make preparations for increasing the size of the kernel virtual
  address space on the amd64 architecture:

  Rev 180333, ``Change create_pagetables() and pmap_init() so that many
  fewer page table pages have to be preallocated ...'', violates an
  assumption made by minidumpsys(): kernel_vm_end is the highest virtual
  address that has ever been used by the kernel.  Now, however, the kernel
  code, data, and bss may reside at addresses beyond kernel_vm_end.  This
  revision modifies the upper bound on minidumpsys()'s two page table
  traversals to account for this possibility.