ip monitor: Skip IPv6 ND user option messages

IPv6 router sends ND messages with RDNSS option
which causes the printing of unknown message by 'ip monitor':

    Unknown message: 0000004c 00000044 00000000
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

ip/ipmonitor.c

@@ -138,7 +138,8 @@ static int accept_msg(const struct sockaddr_nl *who,
 	    n->nlmsg_type == RTM_NEWTCLASS ||
 	    n->nlmsg_type == RTM_DELTCLASS ||
 	    n->nlmsg_type == RTM_NEWTFILTER ||
-	    n->nlmsg_type == RTM_DELTFILTER)
+	    n->nlmsg_type == RTM_DELTFILTER ||
+	    n->nlmsg_type == RTM_NEWNDUSEROPT)
 		return 0;
 	if (n->nlmsg_type != NLMSG_ERROR && n->nlmsg_type != NLMSG_NOOP &&
 	    n->nlmsg_type != NLMSG_DONE) {