vlan: fix a use-after-free in vlan_device_event()

After refcnt reaches zero, vlan_vid_del() could free
dev->vlan_info via RCU:

RCU_INIT_POINTER(dev->vlan_info, NULL);
call_rcu(&vlan_info->rcu, vlan_info_rcu_free);

However, the pointer 'grp' still points to that memory
since it is set before vlan_vid_del():

vlan_info = rtnl_dereference(dev->vlan_info);
if (!vlan_info)
goto out;
grp = &vlan_info->grp;

Depends on when that RCU callback is scheduled, we could
trigger a use-after-free in vlan_group_for_each_dev()
right following this vlan_vid_del().

Fix it by moving vlan_vid_del() before setting grp. This
is also symmetric to the vlan_vid_add() we call in
vlan_device_event().

Reported-by: Fengguang Wu <[e-mail]>
Fixes: efc73f4bbc23 ("net: Fix memory leak - vlan_info struct")
Cc: Alexander Duyck <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Girish Moodalbail <[e-mail]>
Signed-off-by: Cong Wang <[e-mail]>
Reviewed-by: Girish Moodalbail <[e-mail]>
Tested-by: Fengguang Wu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/8021q/vlan.c

···
 			dev->name);
 		vlan_vid_add(dev, htons(ETH_P_8021Q), 0);
 	}
+	if (event == NETDEV_DOWN &&
+	    (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER))
+		vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
 
 	vlan_info = rtnl_dereference(dev->vlan_info);
 	if (!vlan_info)
···
 	case NETDEV_DOWN: {
 		struct net_device *tmp;
 		LIST_HEAD(close_list);
-
-		if (dev->features & NETIF_F_HW_VLAN_CTAG_FILTER)
-			vlan_vid_del(dev, htons(ETH_P_8021Q), 0);
 
 		/* Put all VLANs for this dev in the down state too.  */
 		vlan_group_for_each_dev(grp, i, vlandev) {