cgroup: simplify code in cgroup_apply_control

It could directly return 'cgroup_update_dfl_csses' to simplify code.

Signed-off-by: William Dean <williamsukatube@163.com>
Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com>
Signed-off-by: Tejun Heo <tj@kernel.org>