cgroup: make interface file "cgroup.sane_behavior" legacy-only

"cgroup.sane_behavior" is added to help distinguishing whether
sane_behavior is in effect or not.  We now have the default hierarchy
where the flag is always in effect and are planning to remove
supporting sane behavior on the legacy hierarchies making this file on
the default hierarchy rather pointless.  Let's make it legacy only and
thus always zero.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>