cgroup.c: add helper __cset_cgroup_from_root to cleanup duplicated codes

No funtionality change, but save us some lines.

Signed-off-by: Lin Feng <linf@wangsu.com>
Acked-by: Mukesh Ojha <quic_mojha@quicinc.com>
Signed-off-by: Tejun Heo <tj@kernel.org>