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

net/ipv6/ip6_tunnel.c

@@ -1184,6 +1184,7 @@ 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);
 
 	/* Calculate max headroom for all the headers and adjust
 	 * needed_headroom if necessary.