dma-direct: return appropriate error code from dma_direct_map_sg()

Now that the map_sg() op expects error codes instead of return zero on
error, convert dma_direct_map_sg() to return an error code. Per the
documentation for dma_map_sgtable(), -EIO is returned due to an
DMA_MAPPING_ERROR with unknown cause.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>