In x86 pmap_extract_and_hold()s, handle the case of PHYS_TO_VM_PAGE()
returning NULL.

vm_fault_quick_hold_pages() can be legitimately called on userspace
mappings backed by fictitious pages created by unmanaged device and sg
pagers.

Note that other architectures pmap_extract_and_hold() might need
similar fix, but I postponed the examination.

Reported by: bde
Discussed with: alc
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D16085