blkcg: minor updates around blkcg_policy_data

* Rename blkcg->pd[] to blkcg->cpd[] so that cpd is consistently used
  for blkcg_policy_data.

* Make blkcg_policy->cpd_init_fn() take blkcg_policy_data instead of
  blkcg.  This makes it consistent with blkg_policy_data methods and
  to-be-added cpd alloc/free methods.

* blkcg_policy_data->blkcg and cpd_to_blkcg() added so that
  cpd_init_fn() can determine the associated blkcg from
  blkcg_policy_data.

v2: blkcg_policy_data->blkcg initializations were missing.  Added.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 95e6b0c..dd6ea9e 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -220,7 +220,7 @@
 /* Per-cgroup data */
 struct cfq_group_data {
 	/* must be the first member */
-	struct blkcg_policy_data pd;
+	struct blkcg_policy_data cpd;
 
 	unsigned int weight;
 	unsigned int leaf_weight;
@@ -612,7 +612,7 @@
 static struct cfq_group_data
 *cpd_to_cfqgd(struct blkcg_policy_data *cpd)
 {
-	return cpd ? container_of(cpd, struct cfq_group_data, pd) : NULL;
+	return cpd ? container_of(cpd, struct cfq_group_data, cpd) : NULL;
 }
 
 static inline struct blkcg_gq *cfqg_to_blkg(struct cfq_group *cfqg)
@@ -1568,12 +1568,11 @@
 #endif
 }
 
-static void cfq_cpd_init(const struct blkcg *blkcg)
+static void cfq_cpd_init(struct blkcg_policy_data *cpd)
 {
-	struct cfq_group_data *cgd =
-		cpd_to_cfqgd(blkcg->pd[blkcg_policy_cfq.plid]);
+	struct cfq_group_data *cgd = cpd_to_cfqgd(cpd);
 
-	if (blkcg == &blkcg_root) {
+	if (cpd_to_blkcg(cpd) == &blkcg_root) {
 		cgd->weight = 2 * CFQ_WEIGHT_DEFAULT;
 		cgd->leaf_weight = 2 * CFQ_WEIGHT_DEFAULT;
 	} else {