block: simplify I/O stat accounting

This simplifies I/O stat accounting switching code and separates it
completely from I/O scheduler switch code.

Requests are accounted according to the state of their request queue
at the time of the request allocation. There is no need anymore to
flush the request queue when switching I/O accounting state.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/blk.h b/block/blk.h
index 5dfc412..79c85f7 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -114,12 +114,7 @@
 
 static inline int blk_do_io_stat(struct request *rq)
 {
-	struct gendisk *disk = rq->rq_disk;
-
-	if (!disk || !disk->queue)
-		return 0;
-
-	return blk_queue_io_stat(disk->queue) && (rq->cmd_flags & REQ_ELVPRIV);
+	return rq->rq_disk && blk_rq_io_stat(rq);
 }
 
 #endif