workqueue: add missing POOL_FREEZING

get_unbound_pool() forgot to set POOL_FREEZING if workqueue_freezing
is set and a new pool could go out of sync with the global freezing
state.

Fix it by adding POOL_FREEZING if workqueue_freezing.  wq_mutex is
already held so no further locking is necessary.  This also removes
the unused static variable warning when !CONFIG_FREEZER.

tj: Updated commit message.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3503,6 +3503,9 @@ static struct worker_pool *get_unbound_pool(const struct workqueue_attrs *attrs)
 	if (!pool || init_worker_pool(pool) < 0)
 		goto fail;
 
+	if (workqueue_freezing)
+		pool->flags |= POOL_FREEZING;
+
 	lockdep_set_subclass(&pool->lock, 1);	/* see put_pwq() */
 	copy_workqueue_attrs(pool->attrs, attrs);