api: cgroup_get_cgroup() populate the controller list based on the subtree_control...

api: cgroup_get_cgroup() populate the controller list based on the subtree_control file

In cgroup_get_cgroup(), populate the controller list in the cgroup
only with controllers that are enabled in cgroup.subtree_control.

As part of this change update the error codes from
cgroupv2_get_subtree_control() to better reflect memory allocation
errors

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