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>