ipv6: don't call addrconf_dst_alloc again when enable lo

If we disable all of the net interfaces, and enable
un-lo interface before lo interface, we already allocated
the addrconf dst in ipv6_add_addr. So we shouldn't allocate
it again when we enable lo interface.

Otherwise the message below will be triggered.
unregister_netdevice: waiting for sit1 to become free. Usage count = 1

This problem is introduced by commit 25fb6ca4
"net IPv6 : Fix broken IPv6 routing table after loopback down-up"
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -2655,6 +2655,9 @@ static void init_loopback(struct net_device *dev)
 			if (sp_ifa->flags & (IFA_F_DADFAILED | IFA_F_TENTATIVE))
 				continue;
 
+			if (sp_ifa->rt)
+				continue;
+
 			sp_rt = addrconf_dst_alloc(idev, &sp_ifa->addr, 0);
 
 			/* Failure cases are ignored */