intel-iommu mistakenly uses offset_pfn when caching mode is enabled

intel_map_sg used offset_pfn which was set to zero when invalidating the IOTLB.
intel_map_sg now uses size variable for this matter.

Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>