linux-phytec-fsl

cgroup: simplify threadgroup locking

Now that threadgroup locking is made global, code paths around it can
be simplified.

* lock-verify-unlock-retry dancing removed from __cgroup_procs_write().

* Race protection against de_thread() removed from
  cgroup_update_dfl_csses().

Signed-off-by: Tejun Heo <tj@kernel.org>