cgroup: Add mount flag to enable cpuset to use v2 behavior in v1 cgroup

A new mount option "cpuset_v2_mode" is added to the v1 cgroupfs
filesystem to enable cpuset controller to use v2 behavior in a v1
cgroup. This mount option applies only to cpuset controller and have
no effect on other controllers.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup-defs.h

@@ -74,6 +74,11 @@ enum {
 	 * aren't writeable from inside the namespace.
 	 */
 	CGRP_ROOT_NS_DELEGATE	= (1 << 3),
+
+	/*
+	 * Enable cpuset controller in v1 cgroup to use v2 behavior.
+	 */
+	CGRP_ROOT_CPUSET_V2_MODE = (1 << 4),
 };
 
 /* cftype->flags */