cgroup/cpuset: Reduce the lock protecting CS_SCHED_LOAD_BALANCE

cgroup/cpuset: Reduce the lock protecting CS_SCHED_LOAD_BALANCE

In the cpuset_css_online(), clearing the CS_SCHED_LOAD_BALANCE bit
of cs->flags is guarded by callback_lock and cpuset_mutex. There is
no problem with itself, because it is consistent with the description
of there two global lock at the beginning of this file. However, since
the operation of checking, setting and clearing the flag bit is atomic,
protection of callback_lock is unnecessary here, see CS_SPREAD_*. so
to make it more consistent with the other code, move the operation
outside the critical section of callback_lock.

No functional changes intended.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>