ip6_gre: fix error path when ip6erspan_rcv failed

Same as ipv4 code, when ip6erspan_rcv call return PACKET_REJECT, we
should call icmpv6_send to send icmp unreachable message in error path.

Fixes: 5a963eb6 ("ip6_gre: Add ERSPAN native tunnel support")
Acked-by: NWilliam Tu <u9012063@gmail.com>
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -602,12 +602,13 @@ static int gre_rcv(struct sk_buff *skb)
 		     tpi.proto == htons(ETH_P_ERSPAN2))) {
 		if (ip6erspan_rcv(skb, hdr_len, &tpi) == PACKET_RCVD)
 			return 0;
-		goto drop;
+		goto out;
 	}
 
 	if (ip6gre_rcv(skb, &tpi) == PACKET_RCVD)
 		return 0;
 
+out:
 	icmpv6_send(skb, ICMPV6_DEST_UNREACH, ICMPV6_PORT_UNREACH, 0);
 drop:
 	kfree_skb(skb);