intel-iommu: Fix integer wrap on 32 bit kernels

The following 64 bit promotions are necessary to handle memory above the
4GiB boundary correctly.

[dwmw2: Fix the second part not to need 64-bit arithmetic at all]

Signed-off-by: Benjamin LaHaise <ben.lahaise@neterion.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>