[PATCH cgroup/for-4.15-fixes] cgroup: fix css_task_iter crash on CSS_TASK_ITER_PROC

[Tejun Heo]

Hello,

Applied the following to cgroup/for-4.15-fixes.  Will push out to
linus later this week.  I could reproduce the problem reliably and am
pretty sure this is the right fix but I'd greatly appreciate if you
guys can confirm the fix too.

Thank you very much.

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