md: use generic io stats accounting functions to simplify io stat accounting

Use generic io stats accounting help functions (generic_{start,end}_io_acct)
to simplify io stat accounting.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 9233c71..056ccd2 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -247,7 +247,6 @@
 {
 	const int rw = bio_data_dir(bio);
 	struct mddev *mddev = q->queuedata;
-	int cpu;
 	unsigned int sectors;
 
 	if (mddev == NULL || mddev->pers == NULL
@@ -284,10 +283,7 @@
 	sectors = bio_sectors(bio);
 	mddev->pers->make_request(mddev, bio);
 
-	cpu = part_stat_lock();
-	part_stat_inc(cpu, &mddev->gendisk->part0, ios[rw]);
-	part_stat_add(cpu, &mddev->gendisk->part0, sectors[rw], sectors);
-	part_stat_unlock();
+	generic_start_io_acct(rw, sectors, &mddev->gendisk->part0);
 
 	if (atomic_dec_and_test(&mddev->active_io) && mddev->suspended)
 		wake_up(&mddev->sb_wait);