Revision 187465

MFC r179917,180333,180373,180378
  Prepare for a larger kernel virtual address space.  Specifically, once
  KERNBASE and VM_MIN_KERNEL_ADDRESS are no longer the same, the physical
  memory allocated during bootstrap will be offset from the low-end of the
  kernel's page table.

  Change create_pagetables() and pmap_init() so that many fewer page table
  pages have to be preallocated by create_pagetables().

  Eliminate pmap_growkernel()'s dependence on create_pagetables() preallocating
  page directory pages from VM_MIN_KERNEL_ADDRESS through the end of the
  kernel's bss.  Specifically, the dependence was in pmap_growkernel()'s one-
  time initialization of kernel_vm_end, not in its main body.  (I could not,
  however, resist the urge to optimize the main body.)

  Reduce the number of preallocated page directory pages to just those needed
  to support NKPT page table pages.  (In fact, this allows me to revert a
  couple of my earlier changes to create_pagetables().)

  Fix lines that are too long in pmap_growkernel() by substituting shorter
  but equivalent expressions.