ip6_tunnel: fix setting hop_limit value for ipv6 tunnel

Similar to vxlan/geneve tunnel, if hop_limit is zero, it should fall
back to ip6_dst_hoplimt().

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>