cgroup: Add missing cpus_read_lock() to cgroup_attach_task_all()

stable inclusion
from stable-v4.19.280
commit 321488cfac7d0eb6d97de467015ff754f85813ff
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I6TI3Y
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=321488cfac7d0eb6d97de467015ff754f85813ff

--------------------------------

commit 43626dad upstream.

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.

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

 [1]
Reported-by: syzbot <syzbot+29d3a3b4d86c8136ad9e@syzkaller.appspotmail.com>
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>
Signed-off-by: Cai Xinchen <caixinchen1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

conflicts:
	kernel/cgroup/cgroup-internal.h
	kernel/cgroup/cgroup-v1.c
	kernel/cgroup/cgroup.c

Signed-off-by: Cai Xinchen <caixinchen1@huawei.com>
Reviewed-by: Wang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>