iommu/vt-d: Fix missing IOTLB flush in intel_iommu_unmap()

iommu/vt-d: Fix missing IOTLB flush in intel_iommu_unmap()

Part of commit ea8ea460c9ace60bbb5ac6e5521d637d5c15293d upstream.

This missing IOTLB flush was added as a minor, inconsequential bug-fix
in commit ea8ea460c ("iommu/vt-d: Clean up and fix page table clear/free
behaviour") in 3.15. It wasn't originally intended for -stable but a
couple of users have reported issues which turn out to be fixed by
adding the missing flush.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Use &dmar_domain->iommu_bmp, as it is a single word not an array]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>