Have uiomove_object_page() keep accessed pages in the active queue.

Previously, uiomove_object_page() would maintain LRU by requeuing the
accessed page. This involves acquiring one of the heavily contended page
queue locks. Moreover, it is unnecessarily expensive for pages in the
active queue.

As of r254304 the page daemon continually performs a slow scan of the
active queue, with the effect that unreferenced pages are gradually
moved to the inactive queue, from which they can be reclaimed. Prior to
that revision, the active queue was scanned only during shortages of
free and inactive pages, meaning that unreferenced pages could get
"stuck" in the queue. Thus, tmpfs was required to use the inactive queue
and requeue pages in order to maintain LRU. Now that this is no longer
the case, tmpfs I/O operations can use the active queue and avoid the
page queue locks in most cases, instead setting PGA_REFERENCED on
referenced pages to provide pseudo-LRU.

Reviewed by: alc (previous version)
MFC after: 2 weeks