blkcg: add blkcg_root_css

Add global constant blkcg_root_css which points to &blkcg_root.css.
This will be used by cgroup writeback support.  If blkcg is disabled,
it's defined as ERR_PTR(-EINVAL).

v2: The declarations moved to include/linux/blk-cgroup.h as suggested
    by Vivek.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-cgroup.c

@@ -30,6 +30,8 @@ struct blkcg blkcg_root = { .cfq_weight = 2 * CFQ_WEIGHT_DEFAULT,
 			    .cfq_leaf_weight = 2 * CFQ_WEIGHT_DEFAULT, };
 EXPORT_SYMBOL_GPL(blkcg_root);
 
+struct cgroup_subsys_state * const blkcg_root_css = &blkcg_root.css;
+
 static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];
 
 static bool blkcg_policy_enabled(struct request_queue *q,

include/linux/blk-cgroup.h

@@ -134,6 +134,7 @@ struct blkcg_policy {
 };
 
 extern struct blkcg blkcg_root;
+extern struct cgroup_subsys_state * const blkcg_root_css;
 
 struct blkcg_gq *blkg_lookup(struct blkcg *blkcg, struct request_queue *q);
 struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
@@ -570,6 +571,8 @@ struct blkcg_gq {
 struct blkcg_policy {
 };
 
+#define blkcg_root_css	((struct cgroup_subsys_state *)ERR_PTR(-EINVAL))
+
 #ifdef CONFIG_BLOCK
 
 static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg, void *key) { return NULL; }