udp_tunnel: Seperate ipv6 functions into its own file.

Add ip6_udp_tunnel.c for ipv6 UDP tunnel functions to avoid ifdefs
in udp_tunnel.c

Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/udp_tunnel.h
Simple merge

diff --cc net/ipv4/udp_tunnel.c
Simple merge

diff --cc net/ipv6/Makefile
Simple merge

diff --cc net/ipv6/ip6_udp_tunnel.c
Simple merge