kernel - Adjust tlb invalidation in the x86-64 pmap code

kernel - Adjust tlb invalidation in the x86-64 pmap code

* Use a locked bus cycle instruction to clear pte's in all cases.

* Remove unnecessary vm_page_hold() when removing a page table page pv.
  The page is still wired so a hold is not needed.

* Do not issue invalidation interlocks when populating a user pte, the
  invalidations issued when the user pte is removed are sufficient.

  Kernel pte's still appear to need an interlock.  It is unclear why
  (possibly early PG_PS replacement issues).

* Revamp pmap_enter() to fix a race case which could allow PG_M to get
  lost.  Any protection or wiring change fully removes the pte before
  loading a revised pte.