Extend r335969 to superpages.

It is possible that a fictitious unmanaged userspace mapping of
superpage is created on x86, e.g. by pmap_object_init_pt(), with the
physical address outside the vm_page_array[] coverage.

Noted and reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D16085