btrfs: account as waiting for IO, while waiting fot the flush bio completion

Similar to what submit_bio_wait does, we should account for IO while
waiting for a bio completion. This has marginal visible effects, flush
bio is short-lived.
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -3519,7 +3519,7 @@ static int wait_dev_flush(struct btrfs_device *device)
 		return 0;
 
 	device->flush_bio_sent = 0;
-	wait_for_completion(&device->flush_wait);
+	wait_for_completion_io(&device->flush_wait);
 
 	if (bio->bi_error) {
 		ret = bio->bi_error;