cpuset: Fix unsafe lock order between cpuset lock and cpuslock

The backport commit 4eec5fe1 ("cgroup/cpuset: Fix a race
between cpuset_attach() and cpu hotplug") looks suspicious since
it comes before commit d74b27d6 ("cgroup/cpuset: Change
cpuset_rwsem and hotplug lock order") v5.4-rc1~176^2~30 when
the locking order was: cpuset lock, cpus lock.

Fix it with the correct locking order and reduce the cpus locking
range because only set_cpus_allowed_ptr() needs the protection of
cpus lock.

Fixes: 4eec5fe1 ("cgroup/cpuset: Fix a race between cpuset_attach() and cpu hotplug")
Reported-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>