vm_pageout_scans: correct detection of active object

For non-anonymous swap objects, there is always a reference from the
owner to the object to keep it from recycling. Account for it when
deciding should we query pmap for hardware active references for the
page.

As result, we avoid unneeded calls to pmap_ts_referenced(), which for
non-mapped page means avoiding unneccessary lock and unlock of the pv list.

Reviewed by: markj
Discussed with: alc
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33924