intel-iommu: Fix dma vs. mm page confusion with aligned_nrpages()

The aligned_nrpages() function rounds up to the next VM page, but
returns its result as a number of DMA pages.

Purely theoretical except on IA64, which doesn't boot with VT-d right
now anyway.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>