From: Stephen Hemminger <redacted>
Date: Sat, 27 Dec 2014 10:12:39 -0800

From: Bill Hong <redacted>

Previously l2tp module did not provide any means for the user space to
get notified when tunnels/sessions are added/modified/deleted.
This change contains the following
- create a multicast group for the listeners to register.
- notify the registered listeners when the tunnels/sessions are
  created/modified/deleted.

Signed-off-by: Bill Hong <redacted>
Reviewed-by: Stephen Hemminger <redacted>
Reviewed-by: Sven-Thorsten Dietrich <redacted>

Applied.