cgroup: Move cgrp_dead_ task+iwork into its own struct

cgrp_dead_tasks and cgrp_dead_tasks_iwork are independent per-CPU variables
and are independently accessed via a this_cpu_ptr().

Having a custom struct with those two members makes it possible to have only
one per-CPU accessor and access the second member via an offset. This makes
the code a bit more compact and is a micro-optimization.

Besides that, it allows cgrp_dead_tasks_iwork_fn() to access the list
pointer via the passed iwork pointer. This not only eliminates the
this_cpu_ptr() but also makes it possible to be invoked on a different CPU.
This does not happen as of today but might if I can flush the irq_work items
from another CPU during CPU-hotplug events.

Create struct cgroup_dead with the per-CPU variables as members. Access the
struct with a per-CPU accessor and use container_of() in the irq_work
callback.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>