kernel - Attempt to fix i386 wire_count panic

kernel - Attempt to fix i386 wire_count panic

* Finally found what could be the issue.  get_pv_entry() calls zalloc()
  which can fall through to zget() which obtains a zalloc-related LWKT
  token.

  This can temporarily break the vm_token and allow another thread to get
  in and change the pmap pte entry out from under a pmap_enter(), causing
  the pmap_enter() to potentially remove an extra wire_count from the
  page table page.

* Fix by pre-allocating the pv entry, taking it out of the critical path,
  and adjusting a few other bits of code to test the *pte closer to
  the code which replaces it for the purposes of adjusting the wire_count.