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>