Allow for fictitious physical pages in vm_page_scan_contig().

Some drm2 drivers will set PG_FICTITIOUS in physical pages in order to
satisfy the OBJT_MGTDEVICE object interface, so a scan may encounter
fictitous pages. For now, allow for this possibility; such pages will be
skipped later in the scan since they are wired.

Reported by: avg
Reviewed by: kib
MFC after: 1 week