Do not access pv_table array for fictitious pages, since the array
does not cover the dynamically registered ficititious ranges, and
fictitious pages mappings are not promoted. Offer a dummy struct
md_page to fetch constant superpage pv list generation to satisfy
logic. Also, by initializing the pv_dummy pv_list to empty, we can
remove several explicit PG_FICTITIOUS tests.

Reported and tested by: Michael Butler <imb@protected-networks.net>
(previous version)
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D6728
Approved by: re (hrs)