cgroup: Fix incorrect WARN_ON_ONCE() in css_release_work_fn()

It turns out that the WARN_ON_ONCE() call in css_release_work_fn
introduced by commit ab031252 ("cgroup: Show # of subsystem CSSes
in cgroup.stat") is incorrect. Although css->nr_descendants must be
0 when a css is released and ready to be freed, the corresponding
cgrp->nr_dying_subsys[ss->id] may not be 0 if a subsystem is activated
and deactivated multiple times with one or more of its previous
activation leaving behind dying csses.

Fix the incorrect warning by removing the cgrp->nr_dying_subsys check.

Fixes: ab031252 ("cgroup: Show # of subsystem CSSes in cgroup.stat")
Closes: https://lore.kernel.org/cgroups/6f301773-2fce-4602-a391-8af7ef00b2fb@redhat.com/T/#t


Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>