Accurately reports what happened in icmpv6_notify() when handling
a packet.

This makes use of the new IPV6_BAD_EXTHDR drop reason.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>