cgroup: export list of cgroups v2 features using sysfs

The active development of cgroups v2 sometimes leads to a creation
of interfaces, which are not turned on by default (to provide
backward compatibility). It's handy to know from userspace, which
cgroup v2 features are supported without calculating it based
on the kernel version. So, let's export the list of such features
using /sys/kernel/cgroup/features pseudo-file.

The list is hardcoded and has to be extended when new functionality
is added. Each feature is printed on a new line.

Example:
  $ cat /sys/kernel/cgroup/features
  nsdelegate

Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
Signed-off-by: Tejun Heo <tj@kernel.org>