l2tp: auto load type modules

It should not be necessary to do explicit module loading when
configuring L2TP. Modules should be loaded as needed instead
(as is done already with netlink and other tunnel types).

This patch adds a new module alias type and code to load
the sub module on demand.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index 9e13c2f..f93c5be 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -576,6 +576,13 @@
 	if (info->attrs[L2TP_ATTR_MRU])
 		cfg.mru = nla_get_u16(info->attrs[L2TP_ATTR_MRU]);
 
+#ifdef CONFIG_MODULES
+	if (l2tp_nl_cmd_ops[cfg.pw_type] == NULL) {
+		genl_unlock();
+		request_module("net-l2tp-type-%u", cfg.pw_type);
+		genl_lock();
+	}
+#endif
 	if ((l2tp_nl_cmd_ops[cfg.pw_type] == NULL) ||
 	    (l2tp_nl_cmd_ops[cfg.pw_type]->session_create == NULL)) {
 		ret = -EPROTONOSUPPORT;