Improve get_pv_entry()'s handling of low-memory conditions. After page
allocation fails and pv entries are reclaimed, there may be an unused pv
entry in a pv chunk that survived the reclamation. However, previously,
after reclamation, get_pv_entry() did not look for an unused pv entry in
a surviving pv chunk; it simply retried the page allocation. Now, it
does look for an unused pv entry before retrying the page allocation.

Note: This only applies to RELENG_7. Earlier branches use a different
pv entry allocator.

MFC after: 6 weeks