net: ethernet: mtk_wed: fix use-after-free panic in mtk_wed_setup_tc_block_cb()

When there are multiple ap interfaces on one band and with WED on,
turning the interface down will cause a kernel panic on MT798X.

Previously, cb_priv was freed in mtk_wed_setup_tc_block() without
marking NULL,and mtk_wed_setup_tc_block_cb() didn't check the value, too.

Assign NULL after free cb_priv in mtk_wed_setup_tc_block() and check NULL
in mtk_wed_setup_tc_block_cb().

----------
Unable to handle kernel paging request at virtual address 0072460bca32b4f5
Call trace:
 mtk_wed_setup_tc_block_cb+0x4/0x38
 0xffffffc0794084bc
 tcf_block_playback_offloads+0x70/0x1e8
 tcf_block_unbind+0x6c/0xc8
...
---------

Fixes: 79968444 ("net: ethernet: mtk_wed: introduce wed wo support")
Signed-off-by: Zheng Zhang <everything411@qq.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_wed.c

@@ -2666,14 +2666,15 @@ mtk_wed_setup_tc_block_cb(enum tc_setup_type type, void *type_data, void *cb_pri
 {
 	struct mtk_wed_flow_block_priv *priv = cb_priv;
 	struct flow_cls_offload *cls = type_data;
-	struct mtk_wed_hw *hw = priv->hw;
+	struct mtk_wed_hw *hw = NULL;
 
-	if (!tc_can_offload(priv->dev))
+	if (!priv || !tc_can_offload(priv->dev))
 		return -EOPNOTSUPP;
 
 	if (type != TC_SETUP_CLSFLOWER)
 		return -EOPNOTSUPP;
 
+	hw = priv->hw;
 	return mtk_flow_offload_cmd(hw->eth, cls, hw->index);
 }
 
@@ -2729,6 +2730,7 @@ mtk_wed_setup_tc_block(struct mtk_wed_hw *hw, struct net_device *dev,
 			flow_block_cb_remove(block_cb, f);
 			list_del(&block_cb->driver_list);
 			kfree(block_cb->cb_priv);
+			block_cb->cb_priv = NULL;
 		}
 		return 0;
 	default: