ipv6: set multicast flag on the multicast route

The multicast route ff00::/8 is created with type RTN_UNICAST:

  $ ip -6 -d route
  unicast ::1 dev lo proto kernel scope global metric 256 pref medium
  unicast fe80::/64 dev eth0 proto kernel scope global metric 256 pref medium
  unicast ff00::/8 dev eth0 proto kernel scope global metric 256 pref medium

Set the type to RTN_MULTICAST which is more appropriate.

Fixes: e8478e8 ("net/ipv6: Save route type in rt6_info")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/addrconf.c

@@ -2467,7 +2467,7 @@ static void addrconf_add_mroute(struct net_device *dev)
 		.fc_ifindex = dev->ifindex,
 		.fc_dst_len = 8,
 		.fc_flags = RTF_UP,
-		.fc_type = RTN_UNICAST,
+		.fc_type = RTN_MULTICAST,
 		.fc_nlinfo.nl_net = dev_net(dev),
 		.fc_protocol = RTPROT_KERNEL,
 	};