l2tp: calling the ref() instead of deref()

This is a cut and paste typo.  We call ->ref() a second time instead
of ->deref().
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -1777,7 +1777,7 @@ int l2tp_session_delete(struct l2tp_session *session)
 	if (session->session_close != NULL)
 		(*session->session_close)(session);
 	if (session->deref)
-		(*session->ref)(session);
+		(*session->deref)(session);
 	l2tp_session_dec_refcount(session);
 	return 0;
 }