ipv6: ndisc: RFC-ietf-6man-ra-pref64-09 is now published as RFC8781

See:
  https://www.rfc-editor.org/authors/rfc8781.txt

Cc: Erik Kline <ek@google.com>
Cc: Jen Linkova <furry@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Michael Haro <mharo@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Fixes: c24a77ed ("ipv6: ndisc: add support for 'PREF64' dns64 prefix identifier")
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ndisc.h

@@ -41,7 +41,7 @@ enum {
 	ND_OPT_DNSSL = 31,		/* RFC6106 */
 	ND_OPT_6CO = 34,		/* RFC6775 */
 	ND_OPT_CAPTIVE_PORTAL = 37,	/* RFC7710 */
-	ND_OPT_PREF64 = 38,		/* RFC-ietf-6man-ra-pref64-09 */
+	ND_OPT_PREF64 = 38,		/* RFC8781 */
 	__ND_OPT_MAX
 };