udp: Add udp_sock_create for UDP tunnels to open listener socket

Added udp_tunnel.c which can contain some common functions for UDP
tunnels. The first function in this is udp_sock_create which is used
to open the listener port for a UDP tunnel.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index 05c57f0..dbc10d8 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -307,6 +307,10 @@
 	  the notion of a secure tunnel for IPSEC and then use routing protocol
 	  on top.
 
+config NET_UDP_TUNNEL
+	tristate
+	default n
+
 config INET_AH
 	tristate "IP: AH transformation"
 	select XFRM_ALGO