net-ipv6-ndisc: add support for RFC7710 RA Captive Portal Identifier

net-ipv6-ndisc: add support for RFC7710 RA Captive Portal Identifier

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

As specified in RFC7710, Captive Portal option contains a URL.
8-bit identifier of the option type as assigned by the IANA is 37.
This option should also be treated as userland.

Hence, treat ND option 37 as userland (Captive Portal support)

See:
  https://tools.ietf.org/html/rfc7710
  https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml

Fixes: e35f30c131a56
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Remin Nguyen Van <reminv@google.com>
Cc: Alexey I. Froloff <raorn@raorn.name>
Signed-off-by: David S. Miller <davem@davemloft.net>