gunit/012: populate root_cgroup.controllers file

gunit/012: populate root_cgroup.controllers file

To enable a controller in a cgroup, its parent's subtree_control file is
read to check if the controller is enabled, if not enabled, parent's
cgroup.controllers is checked for the supported controllers, before
enabling it in the cgroup.subtree_control.

Populate controllers in the root_cgroup.controllers file, to help with
the check for supported controllers, without this
cgroupv2_subtree_control_recursive() will fail, when it is checked for
controller existence.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>