block: add a rq_dma_dir helper

In a lot of places we want to know the DMA direction for a given
struct request.  Add a little helper to make it a littler easier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

include/linux/blkdev.h

@@ -641,6 +641,9 @@ static inline bool blk_account_rq(struct request *rq)
 
 #define rq_data_dir(rq)		(op_is_write(req_op(rq)) ? WRITE : READ)
 
+#define rq_dma_dir(rq) \
+	(op_is_write(req_op(rq)) ? DMA_TO_DEVICE : DMA_FROM_DEVICE)
+
 static inline bool queue_is_mq(struct request_queue *q)
 {
 	return q->mq_ops;