ipgre_err() can call ip6_err_gen_icmpv6_unreach() for proper
support of ipv4+gre+icmp+ipv6+... frames, used for example
by traceroute/mtr.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>