kernel - Fix bug in pmap_unwire()
* Fix a bug introduced in the recent pmap work. For kernel pages,
pmap_unwire() was not testing the V bit and would return a bogus
vm_page_t for PTE values of 0, instead of returning NULL.
* Triggered by the mpipe code, used primarily by the crypto subsystem.
Also used for IP fragment reassembly and by tcp_maketemplate(). A
failed M_NOWAIT slab allocation tries to delete the partially filled
map_entry_t, encounters 0 PTEs, and gets confused.
Reported-by: Tim Darby, Antonio Huete Jimenez