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

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: 408687e7b6b2 ("ipv6: ndisc: add support for 'PREF64' dns64 prefix identifier")
Signed-off-by: David S. Miller <davem@davemloft.net>