xen: swiotlb: return error code from xen_swiotlb_map_sg()

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

xen_swiotlb_map_sg() may only fail if xen_swiotlb_map_page() fails, but
xen_swiotlb_map_page() only supports returning errors as
DMA_MAPPING_ERROR. So coalesce all errors into EIO per the documentation
for dma_map_sgtable().

Signed-off-by: Martin Oliveira <martin.oliveira@eideticom.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>