Treat ND option 31 as userland (DNSSL support)

As specified in RFC6106, DNSSL option contains one or more domain names
of DNS suffixes.  8-bit identifier of the DNSSL option type as assigned
by the IANA is 31.  This option should also be treated as userland.

Signed-off-by: Alexey I. Froloff <raorn@raorn.name>
Signed-off-by: David S. Miller <davem@davemloft.net>