ipv6: Fix neigh lookup using NULL device.

In some of the rt6_bind_neighbour() call sites, it hasn't hooked
up the rt->dst.dev pointer yet, so we'd deref a NULL pointer when
obtaining dev->ifindex for the neighbour hash function computation.

Just pass the netdevice explicitly in to fix this problem.

Reported-by: Bjarke Istrup Pedersen <gurligebis@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>