ipv6: Add support for IPsec virtual tunnel interfaces

This patch adds IPv6  support for IPsec virtual tunnel interfaces
(vti). IPsec virtual tunnel interfaces provide a routable interface
for IPsec tunnel endpoints.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/net/ipv6/Kconfig b/net/ipv6/Kconfig
index 11b13ea..e1a8d90 100644
--- a/net/ipv6/Kconfig
+++ b/net/ipv6/Kconfig
@@ -153,6 +153,17 @@
 	---help---
 	  Support for MIPv6 route optimization mode.
 
+config IPV6_VTI
+tristate "Virtual (secure) IPv6: tunneling"
+	select IPV6_TUNNEL
+	depends on INET6_XFRM_MODE_TUNNEL
+	---help---
+	Tunneling means encapsulating data of one protocol type within
+	another protocol and sending it over a channel that understands the
+	encapsulating protocol. This can be used with xfrm mode tunnel to give
+	the notion of a secure tunnel for IPSEC and then use routing protocol
+	on top.
+
 config IPV6_SIT
 	tristate "IPv6: IPv6-in-IPv4 tunnel (SIT driver)"
 	select INET_TUNNEL