Bluetooth: 6lowpan: use after free in disconnect_devices()

This was accidentally changed from list_for_each_entry_safe() to
list_for_each_entry() so now it has a use after free bug.  I've changed
it back.

Fixes: 90305829 ('Bluetooth: 6lowpan: Converting rwlocks to use RCU')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/6lowpan.c

@@ -1383,7 +1383,7 @@ static const struct file_operations lowpan_control_fops = {
 
 static void disconnect_devices(void)
 {
-	struct lowpan_dev *entry, *new_dev;
+	struct lowpan_dev *entry, *tmp, *new_dev;
 	struct list_head devices;
 
 	INIT_LIST_HEAD(&devices);
@@ -1408,7 +1408,7 @@ static void disconnect_devices(void)
 
 	rcu_read_unlock();
 
-	list_for_each_entry(entry, &devices, list) {
+	list_for_each_entry_safe(entry, tmp, &devices, list) {
 		ifdown(entry->netdev);
 		BT_DBG("Unregistering netdev %s %p",
 		       entry->netdev->name, entry->netdev);