net: l2tp_eth: fix kernel panic on rmmod l2tp_eth

[ Upstream commit a06998b88b1651c5f71c0e35f528bf2057188ead ]

We must prevent module unloading if some devices are still attached to
l2tp_eth driver.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Tested-by: Denys Fedoryshchenko <denys@visp.net.lb>
Cc: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/net/l2tp/l2tp_eth.c b/net/l2tp/l2tp_eth.c
index 63fe5f3..7446038 100644
--- a/net/l2tp/l2tp_eth.c
+++ b/net/l2tp/l2tp_eth.c
@@ -167,6 +167,7 @@
 		if (dev) {
 			unregister_netdev(dev);
 			spriv->dev = NULL;
+			module_put(THIS_MODULE);
 		}
 	}
 }
@@ -254,6 +255,7 @@
 	if (rc < 0)
 		goto out_del_dev;
 
+	__module_get(THIS_MODULE);
 	/* Must be done after register_netdev() */
 	strlcpy(session->ifname, dev->name, IFNAMSIZ);