Return DMA_MAPPING_ERROR instead of 0 on a dma mapping failure and let
the core dma-mapping code handle the rest.

Note that the existing code used AMD_IOMMU_MAPPING_ERROR to check from
a 0 return from the IOVA allocator, which is replaced with an explicit
0 as in the implementation and other users of that interface.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NLinus Torvalds <torvalds@linux-foundation.org>