kernel - Improve pmap hinting, improve performance

kernel - Improve pmap hinting, improve performance

* Refactor pm_pvhint into two fields, pm_pvhint_pt and pm_pvhint_pte.
  These are the most common hits.

* Consolidate the pv_entry lookup core into pv_entry_lookup() and
  implementing the double hinting.  Adjust pv_cache() to use the
  new fields.

* Improve pmap_object_init_pt() performance by using the new
  RB_SCAN_NOLK() code and soft-busying the VM pages instead
  of hard-busying them.  If we have to deactivate the page, however,
  we must hard-busy the page.

* Fix vm_prefault_quick() committed recently.  When soft-busying
  VM pages for pmap-entry, we have to fall-back to a hard-busy
  if the page must be moved out of PQ_CACHE.