[IPV6]: Comment MRT6_xxx sockopts in include/linux/in6.h.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

include/linux/in6.h

@@ -260,4 +260,19 @@ struct in6_flowlabel_req
 #define IPV6_PREFER_SRC_CGA		0x0008
 #define IPV6_PREFER_SRC_NONCGA		0x0800
 
+/*
+ * Multicast Routing:
+ * see include/linux/mroute6.h.
+ *
+ * MRT6_INIT			200
+ * MRT6_DONE			201
+ * MRT6_ADD_MIF			202
+ * MRT6_DEL_MIF			203
+ * MRT6_ADD_MFC			204
+ * MRT6_DEL_MFC			205
+ * MRT6_VERSION			206
+ * MRT6_ASSERT			207
+ * MRT6_PIM			208
+ * (reserved)			209
+ */
 #endif