MFC: 174104

MFC: 174104
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.

Approved by:	re (kib), alc