iommu/vt-d: Return the correct dma mask when we are bypassing the IOMMU

We must return a mask covering the full physical RAM when bypassing the
IOMMU mapping. Also, in iommu_need_mapping, we need to check using
dma_direct_get_required_mask to ensure that the device's dma_mask can
cover physical RAM before deciding to bypass IOMMU mapping.

Based on an earlier patch from Christoph Hellwig.

Fixes: 249baa54 ("dma-mapping: provide a better default ->get_required_mask")
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>