- Modify pmap_is_current() to return FALSE when a pmap's page table is in

 - Modify pmap_is_current() to return FALSE when a pmap's page table is in
   use because a kernel thread is borrowing it.  The borrowed page table
   can change spontaneously, making any dependence on its continued use
   subject to a race condition.
 - _pmap_unwire_pte_hold() cannot use pmap_is_current(): If a change is
   made to a page table page mapping for a borrowed page table, the TLB
   must be updated.

In collaboration with:	tegge