workqueue: use "pool->cpu < 0" to stand for an unbound pool

There is a piece of sanity checks code in the put_unbound_pool().
The meaning of this code is "if it is not an unbound pool, it will complain
and return" IIUC. But the code uses "pool->flags & POOL_DISASSOCIATED"
imprecisely due to a non-unbound pool may also have this flags.

We should use "pool->cpu < 0" to stand for an unbound pool, so we covert the
code to it.

There is no strictly wrong if we still keep "pool->flags & POOL_DISASSOCIATED"
here, but it is just a noise if we keep it:
  1) we focus on "unbound" here, not "[dis]association".
  2) "pool->cpu < 0" already implies "pool->flags & POOL_DISASSOCIATED".
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3457,7 +3457,7 @@ static void put_unbound_pool(struct worker_pool *pool)
 		return;
 
 	/* sanity checks */
-	if (WARN_ON(!(pool->flags & POOL_DISASSOCIATED)) ||
+	if (WARN_ON(!(pool->cpu < 0)) ||
 	    WARN_ON(!list_empty(&pool->worklist)))
 		return;