cgroup: add css_set->dfl_cgrp

To implement the unified hierarchy behavior, we'll need to be able to
determine the associated cgroup on the default hierarchy from css_set.
Let's add css_set->dfl_cgrp so that it can be accessed conveniently
and efficiently.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 18fcae3..c49d161 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -354,6 +354,9 @@
 	 */
 	struct list_head cgrp_links;
 
+	/* the default cgroup associated with this css_set */
+	struct cgroup *dfl_cgrp;
+
 	/*
 	 * Set of subsystem states, one for each subsystem. This array is
 	 * immutable after creation apart from the init_css_set during