l2tp: fix userspace reception on plain L2TP sockets

As pppol2tp_recv() never queues up packets to plain L2TP sockets,
pppol2tp_recvmsg() never returns data to userspace, thus making
the recv*() system calls unusable.

Instead of dropping packets when the L2TP socket isn't bound to a PPP
channel, this patch adds them to its reception queue.
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -254,13 +254,15 @@ static void pppol2tp_recv(struct l2tp_session *session, struct sk_buff *skb, int
 		po = pppox_sk(sk);
 		ppp_input(&po->chan, skb);
 	} else {
-		l2tp_info(session, PPPOL2TP_MSG_DATA, "%s: socket not bound\n",
-			  session->name);
+		l2tp_dbg(session, PPPOL2TP_MSG_DATA,
+			 "%s: recv %d byte data frame, passing to L2TP socket\n",
+			 session->name, data_len);
 
-		/* Not bound. Nothing we can do, so discard. */
+		if (sock_queue_rcv_skb(sk, skb) < 0) {
 			atomic_long_inc(&session->stats.rx_errors);
 			kfree_skb(skb);
 		}
+	}
 
 	return;