l2tp: fix oops in l2tp_eth_create() error path

When creating an L2TPv3 Ethernet session, if register_netdev() should fail for
any reason (for example, automatic naming for "l2tpeth%d" interfaces hits the
32k-interface limit), the netdev is freed in the error path.  However, the
l2tp_eth_sess structure's dev pointer is left uncleared, and this results in
l2tp_eth_delete() then attempting to unregister the same netdev later in the
session teardown.  This results in an oops.

To avoid this, clear the session dev pointer in the error path.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_eth.c

@@ -291,6 +291,7 @@ static int l2tp_eth_create(struct net *net, u32 tunnel_id, u32 session_id, u32 p
 
 out_del_dev:
 	free_netdev(dev);
+	spriv->dev = NULL;
 out_del_session:
 	l2tp_session_delete(session);
 out: