Eliminate pointless requeueing of pages from terminated objects. These
pages will have left the inactive queue before the page daemon performs
its next scan. Also, ignore references to pages from terminated objects.
This allows the clean pages to be freed a little sooner.

Move some comments to their proper place, i.e., next to the code that
they describe, and update other nearby comments.

Reviewed by: kib
Sponsored by: EMC / Isilon Storage Division