cgroup: Remove data-race around cgrp_dfl_visible

There's a seemingly harmless data-race around cgrp_dfl_visible detected by
kernel concurrency sanitizer. Let's remove it by throwing WRITE/READ_ONCE at
it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Cc: Gabriel Ryan <gabe@cs.columbia.edu>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Link: https://lore.kernel.org/netdev/20220819072256.fn7ctciefy4fc4cu@wittgenstein/