ipv6: ndisc: add support for 'PREF64' dns64 prefix identifier

This is trivial since we already have support for the entirely
identical (from the kernel's point of view) RDNSS, DNSSL, etc. that
also contain opaque data that needs to be passed down to userspace
for further processing.

As specified in draft-ietf-6man-ra-pref64-09 (while it is still a draft,
it is purely waiting on the RFC Editor for cleanups and publishing):
  PREF64 option contains lifetime and a (up to) 96-bit IPv6 prefix.

The 8-bit identifier of the option type as assigned by the IANA is 38.

Since we lack DNS64/NAT64/CLAT support in kernel at the moment,
thus this option should also be passed on to userland.

See:
  https://tools.ietf.org/html/draft-ietf-6man-ra-pref64-09
  https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml#icmpv6-parameters-5

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>
Acked-By: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>