ICMPV6: add support for RFC 8335 PROBE

Add definitions for the ICMPV6 type of Extended Echo Request and
Extended Echo Reply, as defined by sections 2 and 3 of RFC 8335.

Signed-off-by: Andreas Roeseler <andreas.a.roeseler@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>