UPSTREAM: cgroup: Add missing cpus_read_lock() to cgroup_attach_task_all()

syzbot is hitting percpu_rwsem_assert_held(&cpu_hotplug_lock) warning at
cpuset_attach() [1], for commit 4f7e7236 ("cgroup: Fix
threadgroup_rwsem <-> cpus_read_lock() deadlock") missed that
cpuset_attach() is also called from cgroup_attach_task_all().
Add cpus_read_lock() like what cgroup_procs_write_start() does.

Bug:254143784

Link: https://syzkaller.appspot.com/bug?extid=29d3a3b4d86c8136ad9e

 [1]
Reported-by: syzbot <syzbot+29d3a3b4d86c8136ad9e@syzkaller.appspotmail.com>
Change-Id: I21d7ca425c91efe5773ad5e9aec2bbf1c09737f5
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 4f7e7236 ("cgroup: Fix threadgroup_rwsem <-> cpus_read_lock() deadlock")
Signed-off-by: Tejun Heo <tj@kernel.org>
(cherry picked from commit 43626dad
git: //git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git)
Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>