There exists a possible sequence of page table page allocation failures
starting with a superpage demotion by pmap_enter() that could result in
a PV list lock being held when pmap_enter() is just about to return
KERN_RESOURCE_SHORTAGE. Consequently, the KASSERT that no PV list locks
are held needs to be replaced with a conditional unlock.

Discussed with: kib
X-MFC with: r269728
Sponsored by: EMC / Isilon Storage Division