Propagate attribute changes during demotion.

After r349117 and r349122, some mapping attribute changes do not trigger
superpage demotion. However, pmap_demote_l2() was not updated to ensure
that the replacement L3 entries carry any attribute changes that
occurred since promotion.

Reported and tested by: manu
Reviewed by: alc
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20965