Copyout(9) on 4/4 i386 needs correct vm_page_array[].

On the 4/4 i386, copyout(9) may need to call pmap_extract_and_hold()
on arbitrary userspace mapping. If the mapping is backed by the
non-managed cdev pager or by the sg pager, on dense configs we might
access arbitrary element of vm_page_array[], in particular, not
corresponding to a page from the memory segment. Initialize such pages
as fictitious with the corresponding physical address.

Reported by: bde
Reviewed by: alc, markj (previous version)
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D16085