[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: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>