blk-stat: kill blk_stat_rq_ddir()

No point in providing and exporting this helper. There's just
one (real) user of it, just use rq_data_dir().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index fa7d86f..0b2f604 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -46,7 +46,7 @@ static int blk_mq_poll_stats_bkt(const struct request *rq)
 {
 	int ddir, bytes, bucket;
 
-	ddir = blk_stat_rq_ddir(rq);
+	ddir = rq_data_dir(rq);
 	bytes = blk_rq_bytes(rq);
 
 	bucket = ddir + 2*(ilog2(bytes) - 9);