IB/mlx5: Flush cache workqueue before destroying it

Destroying the workqueue without flushing it first can lead to a case
in which the kernel tries to push a delayed work to the workqueue
which does not exist anymore.
Signed-off-by: NMoshe Lazer <moshel@mellanox.com>
Signed-off-by: NEli Cohen <eli@mellanox.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>

drivers/infiniband/hw/mlx5/mr.c

@@ -415,6 +415,7 @@ static void clean_keys(struct mlx5_ib_dev *dev, int c)
 	int size;
 	int err;
 
+	cancel_delayed_work(&ent->dwork);
 	while (1) {
 		spin_lock(&ent->lock);
 		if (list_empty(&ent->head)) {
@@ -540,13 +541,15 @@ int mlx5_mr_cache_cleanup(struct mlx5_ib_dev *dev)
 	int i;
 
 	dev->cache.stopped = 1;
-	destroy_workqueue(dev->cache.wq);
+	flush_workqueue(dev->cache.wq);
 
 	mlx5_mr_cache_debugfs_cleanup(dev);
 
 	for (i = 0; i < MAX_MR_CACHE_ENTRIES; i++)
 		clean_keys(dev, i);
 
+	destroy_workqueue(dev->cache.wq);
+
 	return 0;
 }