l2tp: add session reorder queue purge function to core

If an l2tp session is deleted, it is necessary to delete skbs in-flight
on the session's reorder queue before taking it down.

Rather than having each pseudowire implementation reaching into the
l2tp_session struct to handle this itself, provide a function in l2tp_core to
purge the session queue.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index e841ef2..69c316d 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -829,6 +829,23 @@
 }
 EXPORT_SYMBOL(l2tp_recv_common);
 
+/* Drop skbs from the session's reorder_q
+ */
+int l2tp_session_queue_purge(struct l2tp_session *session)
+{
+	struct sk_buff *skb = NULL;
+	BUG_ON(!session);
+	BUG_ON(session->magic != L2TP_SESSION_MAGIC);
+	while ((skb = skb_dequeue(&session->reorder_q))) {
+		atomic_long_inc(&session->stats.rx_errors);
+		kfree_skb(skb);
+		if (session->deref)
+			(*session->deref)(session);
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(l2tp_session_queue_purge);
+
 /* Internal UDP receive frame. Do the real work of receiving an L2TP data frame
  * here. The skb is not on a list when we get here.
  * Returns 0 if the packet was a data packet and was successfully passed on.