[IPV6]: Fix [Bug 5306] Oops on IPv6 route lookup

> Steps to reproduce:
> 1. Boot Linux, do NOT setup any IPv6 routes
> 2. ip route get 2001::1 (or any unroutable address)

Well caught.  We never set rt6i_idev on ip6_null_entry.
This patch should make the problem go away.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Acked-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -3520,6 +3520,8 @@ int __init addrconf_init(void)
 	if (err)
 		return err;
 
+	ip6_null_entry.rt6i_idev = in6_dev_get(&loopback_dev);
+
 	register_netdevice_notifier(&ipv6_dev_notf);
 
 #ifdef CONFIG_IPV6_PRIVACY