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.