ipv6: Use ERR_CAST in addrconf_dst_alloc.

	if (IS_ERR(neigh)) {

		dst_free(&rt->dst);

		/* We are casting this because that is the return

		 * value type.  But an errno encoded pointer is the

		 * same regardless of the underlying pointer type,

		 * and that's what we are returning.  So this is OK.

		 */

		return (struct rt6_info *) neigh;

		return ERR_CAST(neigh);

	}

	rt->rt6i_nexthop = neigh;