DMAR pagetables pages: handle ref_count special references

Preserve object reference and block bit during refcount arithmetic
and obliteration, by using vm_page_wire()/unwire_noq().

Reviewed by: markj
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D45910