block: allow queue dma_alignment of zero

Let queue_dma_alignment return 0 if it was specifically set to 0.
This permits devices with no particular alignment restrictions to
use arbitrary user space buffers without copying.
Signed-off-by: NPete Wyckoff <pw@osc.edu>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

include/linux/blkdev.h

@@ -836,12 +836,7 @@ static inline int bdev_hardsect_size(struct block_device *bdev)
 
 static inline int queue_dma_alignment(struct request_queue *q)
 {
-	int retval = 511;
-
-	if (q && q->dma_alignment)
-		retval = q->dma_alignment;
-
-	return retval;
+	return q ? q->dma_alignment : 511;
 }
 
 /* assumes size > 256 */