6pack: Fix use after free in sixpack_close().

Need to do the unregister_device() after all references to the driver
private have been done.

Also we need to use del_timer_sync() for the timers so that we don't
have any asynchronous references after the unregister.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hamradio/6pack.c

@@ -683,14 +683,14 @@ static void sixpack_close(struct tty_struct *tty)
 	if (!atomic_dec_and_test(&sp->refcnt))
 		down(&sp->dead_sem);
 
-	unregister_netdev(sp->dev);
-
-	del_timer(&sp->tx_t);
-	del_timer(&sp->resync_t);
+	del_timer_sync(&sp->tx_t);
+	del_timer_sync(&sp->resync_t);
 
 	/* Free all 6pack frame buffers. */
 	kfree(sp->rbuff);
 	kfree(sp->xbuff);
+
+	unregister_netdev(sp->dev);
 }
 
 /* Perform I/O control on an active 6pack channel. */