Delegatable cgroup v2 control files may require special handling
(e.g. chowning), and the exact list of such files varies between
kernel versions (and likely to be extended in the future).

To guarantee correctness of this list and simplify the life
of userspace (systemd, first of all), let's export the list
via /sys/kernel/cgroup/delegate pseudo-file.

Format is siple: each control file name is printed on a new line.
Example:
  $ cat /sys/kernel/cgroup/delegate
  cgroup.procs
  cgroup.subtree_control
Signed-off-by: NRoman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
Signed-off-by: NTejun Heo <tj@kernel.org>