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

net/ipv6/ip6_tunnel.c

@@ -1188,7 +1188,15 @@ int ip6_tnl_xmit(struct sk_buff *skb, struct net_device *dev, __u8 dsfield,
 		init_tel_txopt(&opt, encap_limit);
 		ipv6_push_frag_opts(skb, &opt.ops, &proto);
 	}
-	hop_limit = hop_limit ? : ip6_dst_hoplimit(dst);
+
+	if (hop_limit == 0) {
+		if (skb->protocol == htons(ETH_P_IP))
+			hop_limit = ip_hdr(skb)->ttl;
+		else if (skb->protocol == htons(ETH_P_IPV6))
+			hop_limit = ipv6_hdr(skb)->hop_limit;
+		else
+			hop_limit = ip6_dst_hoplimit(dst);
+	}
 
 	/* Calculate max headroom for all the headers and adjust
 	 * needed_headroom if necessary.