When mdstart_swap() accesses a page that is already in the active queue,
mark the page as referenced rather than calling vm_page_activate(). This
allows the page's act_count to grow beyond ACT_INIT and better reflect
its usage. (See also r324146, which modified a function used by tmpfs,
uiomove_object_page(), to behave in the same way.)

Reviewed by: kib, markj
MFC after: 2 weeks