Specifically panic() in the case where pmap_insert_entry() fails to
get a new pv under high system load where the available pv entries
have been exhausted before the pagedaemon has a chance to wake up
to reclaim some.

Prior to this, the NULL pointer dereference ended up causing
secondary panics with rather less than useful resulting tracebacks.

Reviewed by: alc, jhb
MFC after: 1 week