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>
diff --git a/include/net/ndisc.h b/include/net/ndisc.h
index 6f9c25a..c02b6ad 100644
--- a/include/net/ndisc.h
+++ b/include/net/ndisc.h
@@ -34,6 +34,7 @@
 	__ND_OPT_ARRAY_MAX,
 	ND_OPT_ROUTE_INFO = 24,		/* RFC4191 */
 	ND_OPT_RDNSS = 25,		/* RFC5006 */
+	ND_OPT_DNSSL = 31,		/* RFC6106 */
 	__ND_OPT_MAX
 };