Do not access pv_table array for fictitious pages, since the array

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)