ip6_tunnel: respect ttl inherit for ip6tnl

man ip-tunnel ttl section says:
0 is a special value meaning that packets inherit the TTL value.

IPv4 tunnel respect this in ip_tunnel_xmit(), but IPv6 tunnel has not
implement it yet. To make IPv6 behave consistently with IP tunnel,
add ipv6 tunnel inherit support.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>