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_core.h b/net/l2tp/l2tp_core.h
index 68aa9ff..5871537 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -321,4 +321,7 @@
 #define l2tp_dbg(ptr, type, fmt, ...)					\
 	l2tp_printk(ptr, type, pr_debug, fmt, ##__VA_ARGS__)
 
+#define MODULE_ALIAS_L2TP_PWTYPE(type) \
+	MODULE_ALIAS("net-l2tp-type-" __stringify(type))
+
 #endif /* _L2TP_CORE_H_ */