ip6tnl,gre6,vti6: implement ndo_get_iflink

Don't use dev->iflink anymore.

CC: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/ip6_tunnel.h
Simple merge

diff --cc net/ipv6/ip6_gre.c
Simple merge

diff --cc net/ipv6/ip6_tunnel.c
Simple merge

diff --cc net/ipv6/ip6_vti.c
Simple merge