cgroup: Remove wrong comment on cgroup_enable_task_cg_list()

Remove the stale comment about RCU protection. Many callers
(all of them?) of cgroup_enable_task_cg_list() don't seem
to be in an RCU read side critical section. Besides, RCU is
not helpful to protect against while_each_thread().

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mandeep Singh Baines <msb@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/cgroup.c

@@ -2701,9 +2701,6 @@ static void cgroup_advance_iter(struct cgroup *cgrp,
  * using their cgroups capability, we don't maintain the lists running
  * through each css_set to its tasks until we see the list actually
  * used - in other words after the first call to cgroup_iter_start().
- *
- * The tasklist_lock is not held here, as do_each_thread() and
- * while_each_thread() are protected by RCU.
  */
 static void cgroup_enable_task_cg_lists(void)
 {