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>

include/uapi/linux/icmpv6.h

@@ -140,6 +140,9 @@ struct icmp6hdr {
 #define ICMPV6_UNK_OPTION		2
 #define ICMPV6_HDR_INCOMP		3
 
+/* Codes for EXT_ECHO (PROBE) */
+#define ICMPV6_EXT_ECHO_REQUEST		160
+#define ICMPV6_EXT_ECHO_REPLY		161
 /*
  *	constants for (set|get)sockopt
  */