dma-mapping: provide a better default ->get_required_mask

Most dma_map_ops instances are IOMMUs that work perfectly fine in 32-bits
of IOVA space, and the generic direct mapping code already provides its
own routines that is intelligent based on the amount of memory actually
present.  Wire up the dma-direct routine for the ARM direct mapping code
as well, and otherwise default to the constant 32-bit mask.  This way
we only need to override it for the occasional odd IOMMU that requires
64-bit IOVA support, or IOMMU drivers that are more efficient if they
can fall back to the direct mapping.

Signed-off-by: Christoph Hellwig <hch@lst.de>

arch/x86/kernel/amd_gart_64.c

@@ -680,6 +680,7 @@ static const struct dma_map_ops gart_dma_ops = {
 	.mmap				= dma_common_mmap,
 	.get_sgtable			= dma_common_get_sgtable,
 	.dma_supported			= dma_direct_supported,
+	.get_required_mask		= dma_direct_get_required_mask,
 };
 
 static void gart_iommu_shutdown(void)