Eliminate spurious "Approaching the limit on PV entries, ..."
warnings. Specifically, whenever vm_page_alloc(9) returned NULL to
get_pv_entry(), we issued a warning regardless of the number of pv
entries in use. (Note: The older pv entry allocator in RELENG_6 does
not have this problem.)

Reported by: Jeremy Chadwick

Eliminate the direct call to pagedaemon_wakeup() by get_pv_entry().
This was a holdover from earlier times when the page daemon was
responsible for the reclamation of pv entries.

MFC after: 5 days