rcu: Make rcutree_dying_cpu() use its "cpu" parameter

The CPU-hotplug functions take a "cpu" parameter, but rcutree_dying_cpu()
ignores it in favor of this_cpu_ptr().  This works at the moment, but
it would be better to be consistent.  This might also work better given
some possible future changes.  This commit therefore uses per_cpu_ptr()
to avoid ignoring the rcutree_dying_cpu() function's argument.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>