dma-mapping: disallow .map_sg operations from returning zero on error

Now that all the .map_sg operations have been converted to returning
proper error codes, drop the code to handle a zero return value,
add a warning if a zero is returned.
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/mapping.c

@@ -197,7 +197,7 @@ static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
 	if (ents > 0)
 		debug_dma_map_sg(dev, sg, nents, ents, dir);
 	else if (WARN_ON_ONCE(ents != -EINVAL && ents != -ENOMEM &&
-			      ents != -EIO && ents != 0))
+			      ents != -EIO))
 		return -EIO;
 
 	return ents;
@@ -261,8 +261,6 @@ int dma_map_sgtable(struct device *dev, struct sg_table *sgt,
 	int nents;
 
 	nents = __dma_map_sg_attrs(dev, sgt->sgl, sgt->orig_nents, dir, attrs);
-	if (nents == 0)
-		return -EIO;
 	if (nents < 0)
 		return nents;
 	sgt->nents = nents;