The .map_sg() op now expects an error code instead of zero on failure.

The only errno to return is -EINVAL in the case when DMA is not
supported.
Signed-off-by: Martin Oliveira <martin.oliveira@eideticom.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>