iommu_gas: Eliminate a possible case of use-after-free

Eliminate a possible case of use-after-free in an error handling path
after a mapping failure. Specifically, eliminate IOMMU_MAP_ENTRY_QI_NF
and instead perform the IOTLB invalidation synchronously. Otherwise,
when iommu_domain_unload_entry() is called and told not to free the
IOMMU map entry, the caller could free the entry before dmar_qi_task()
is finished with it.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35878