From: Anton Blanchard <anton@samba.org>

From: Stephen Rothwell <sfr@canb.auug.org.au>

This creates DMA_ERROR_CODE and uses it everywhere instead of
PCI_DMA_ERROR_CODE as we really want the three DMA mapping API's to return
a single error code.  Also we now have dma_mapping_error and
vio_dma_mapping_error - and this latter and pci_dma_mapping_error both just
call the former.

Also a small fix in the vscsi - dma_map_sg returns 0 to indicate an error.