Replace an unnecessary call to vm_page_activate() by an assertion that
the page is already wired or queued. Prior to the elimination of PG_CACHED
pages, vm_page_grab() might have returned a valid, previously PG_CACHED
page, in which case enqueueing the page was necessary. Now, that can't
happen. Moreover, activating the page is a dubious choice, since the page
is not being accessed.

Reviewed by: kib
MFC after: 1 week