iommu/vt-d: correct flush_unmaps pfn usage

Change flush_unmaps() to correctly pass iommu_flush_iotlb_psi()
dma addresses.  (x86_64 mm and dma have the same size for pages
at the moment, but this usage improves consistency.)

Signed-off-by: Omer Peleg <omer@cs.technion.ac.il>
[mad@cs.technion.ac.il: rebased and reworded the commit message]
Signed-off-by: Adam Morrison <mad@cs.technion.ac.il>
Reviewed-by: Shaohua Li <shli@fb.com>
Reviewed-by: Ben Serebrin <serebrin@google.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>