blkcg: add blkio_policy_ops operations for exit and stat reset

Add blkio_policy_ops->blkio_exit_group_fn() and
->blkio_reset_group_stats_fn().  These will be used to further
modularize blkcg policy implementation.

Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index c82de47..d0ee649 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -153,9 +153,13 @@
 };
 
 typedef void (blkio_init_group_fn)(struct blkio_group *blkg);
+typedef void (blkio_exit_group_fn)(struct blkio_group *blkg);
+typedef void (blkio_reset_group_stats_fn)(struct blkio_group *blkg);
 
 struct blkio_policy_ops {
 	blkio_init_group_fn *blkio_init_group_fn;
+	blkio_exit_group_fn *blkio_exit_group_fn;
+	blkio_reset_group_stats_fn *blkio_reset_group_stats_fn;
 };
 
 struct blkio_policy_type {