ipv6: assign rt6_info to inet6_ifaddr in init_loopback

Commit 25fb6ca4
"net IPv6 : Fix broken IPv6 routing table after loopback down-up"
forgot to assign rt6_info to the inet6_ifaddr.
When disable the net device, the rt6_info which allocated
in init_loopback will not be destroied in __ipv6_ifa_notify.

This will trigger the waring message below
[23527.916091] unregister_netdevice: waiting for tap0 to become free. Usage count = 1
Reported-by: NArkadiusz Miskiewicz <a.miskiewicz@gmail.com>
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -2658,8 +2658,10 @@ static void init_loopback(struct net_device *dev)
 			sp_rt = addrconf_dst_alloc(idev, &sp_ifa->addr, 0);
 
 			/* Failure cases are ignored */
-			if (!IS_ERR(sp_rt))
+			if (!IS_ERR(sp_rt)) {
+				sp_ifa->rt = sp_rt;
 				ip6_ins_rt(sp_rt);
+			}
 		}
 		read_unlock_bh(&idev->lock);
 	}