blkcg: add blkg_policy_data->plid

Add pd->plid so that the policy a pd belongs to can be identified
easily.  This will be used to implement hierarchical blkg_[rw]stats.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 2446225..40f5b97 100644
--- a/block/blk-cgroup.h
+++ b/block/blk-cgroup.h
@@ -81,8 +81,9 @@
  * beginning and pd_size can't be smaller than pd.
  */
 struct blkg_policy_data {
-	/* the blkg this per-policy data belongs to */
+	/* the blkg and policy id this per-policy data belongs to */
 	struct blkcg_gq			*blkg;
+	int				plid;
 
 	/* used during policy activation */
 	struct list_head		alloc_node;