drbd: add missing part_round_stats to _drbd_start_io_acct

Without this, iostat frequently sees bogus svctime and >= 100% "utilization".
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_req.c

@@ -39,6 +39,7 @@ static void _drbd_start_io_acct(struct drbd_conf *mdev, struct drbd_request *req
 	const int rw = bio_data_dir(bio);
 	int cpu;
 	cpu = part_stat_lock();
+	part_round_stats(cpu, &mdev->vdisk->part0);
 	part_stat_inc(cpu, &mdev->vdisk->part0, ios[rw]);
 	part_stat_add(cpu, &mdev->vdisk->part0, sectors[rw], bio_sectors(bio));
 	(void) cpu; /* The macro invocations above want the cpu argument, I do not like