workqueue: Protects wq_unbound_cpumask with wq_pool_attach_mutex

workqueue: Protects wq_unbound_cpumask with wq_pool_attach_mutex

[ Upstream commit 7263795597cd39fed36f0413b513c87e4b1b0e96 ]

When unbind_workers() reads wq_unbound_cpumask to set the affinity of
freshly-unbound kworkers, it only holds wq_pool_attach_mutex. This isn't
sufficient as wq_unbound_cpumask is only protected by wq_pool_mutex.

Make wq_unbound_cpumask protected with wq_pool_attach_mutex and also
remove the need of temporary saved_cpumask.

Fixes: 264fd0e7caf0 ("workqueue: Restrict kworker in the offline CPU pool running on housekeeping CPUs")
Reported-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>