kernel - Fix i386 wire_count panics

kernel - Fix i386 wire_count panics

* Tracked down to a situation where a pmap structure is being dtor'd by
  the objcache simultaniously with a vm_page_protect() operation on
  a page table page's vm_page_t.

  (1) vm_page_protect() begins running, finds page table page to remove,
      removes the related pv_entry, but then gets stuck waiting for the
      pmap->pm_pteobj (vm_object token).

  (2) Exit on another thread simultaniously removes all remaining VM
      pages from the pmap.  However, due to #(1), there is still an
      active page table page in pmap->pm_pteobj that the exit code has
      no visibility to.

  (3) The related pmap is then dtor'd due to heavy fork/exec/exit load
      on the system.  The VM page is still present, vm_page_protect()
      is still stuck on the token (or hasn't gotten cpu back).

  (4) Nominal vm_object_terminate() destroys the page table page.

  (5) vm_page_protect() unblocks and tries to destroy the page.

  (6) BOOM.

* This fix places a barrier between the normal process exit code and the
  dtor which will block while a vm_page_protect() is active on the pmap.

* This time for sure, but if not we still know that the problem is related
  to this exit race.