vm_iommu_map()/unmap(): stop transiently wiring already wired pages

Namely, switch from vm_fault_quick_hold() to pmap_extract() KPI to
translate gpa to hpa. Assert that the looked up hpa belongs to the wired
page, as it should be for the VM which is configured for pass-throu
(this is theoretically a restriction that could be removed on newer
DMARs).

Noted by: alc
Reviewed by: alc, jhb, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D43140