Several minor fixes:
- When freeing pages, it is a good idea to protect them off.
(This is probably gratuitious, but good form.)
- Allow collapsing pages in the backing object that are
PQ_CACHE. This will improve memory utilization.
- Correct the collapse code so that pages that were on the
cache queue are moved to the inactive queue. This is done when pages are marked dirty (so that those pages will be properly paged out instead of freed), so that cached pages will not be paradoxically marked dirty.