block: re-use existing 'reading' variable instead of checking direction again

Signed-off-by: majianpeng <majianpeng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-map.c

@@ -311,7 +311,7 @@ int blk_rq_map_kern(struct request_queue *q, struct request *rq, void *kbuf,
 	if (IS_ERR(bio))
 		return PTR_ERR(bio);
 
-	if (rq_data_dir(rq) == WRITE)
+	if (!reading)
 		bio->bi_rw |= REQ_WRITE;
 
 	if (do_copy)