dm: don't return errnos from ->map

Instead use the special DM_MAPIO_KILL return value to return -EIO just
like we do for the request based path.  Note that dm-log-writes returned
-ENOMEM in a few places, which now becomes -EIO instead.  No consumer
treats -ENOMEM special so this shouldn't be an issue (and it should
use a mempool to start with to make guaranteed progress).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/dm-zero.c b/drivers/md/dm-zero.c
index b616f11..b65ca8d 100644
--- a/drivers/md/dm-zero.c
+++ b/drivers/md/dm-zero.c
@@ -39,7 +39,7 @@ static int zero_map(struct dm_target *ti, struct bio *bio)
 	case REQ_OP_READ:
 		if (bio->bi_opf & REQ_RAHEAD) {
 			/* readahead of null bytes only wastes buffer cache */
-			return -EIO;
+			return DM_MAPIO_KILL;
 		}
 		zero_fill_bio(bio);
 		break;
@@ -47,7 +47,7 @@ static int zero_map(struct dm_target *ti, struct bio *bio)
 		/* writes get silently dropped */
 		break;
 	default:
-		return -EIO;
+		return DM_MAPIO_KILL;
 	}
 
 	bio_endio(bio);