kernel - Greatly reduce kernel memory use for x86_64

kernel - Greatly reduce kernel memory use for x86_64

* Remove the kmemusage array entirely and instead store ku_pagecnt
  in the vm_page structure.

  This allows x86_64 to boot with 128G of KVM without eating tons of
  real memory, in addition to another recent commit which reduced
  real memory use w/ large KVM spaces.

Reported-by: John Marino