Modify reservation resource selection to optimize placement

When a new reservation is created, the code tries first with current
idle nodes:

    /* First: Try to reserve nodes that are currently IDLE */
    if (bit_overlap(avail_bitmap, idle_node_bitmap) >= node_cnt) {
        bit_and(avail_bitmap, idle_node_bitmap);
        ret_bitmap = select_g_resv_test(avail_bitmap, node_cnt,
resv_desc_ptr->core_cnt, core_bitmap);
        if (ret_bitmap)
            goto fini;
    }

    /* Second: Try to reserve nodes that are will be IDLE */

I guess there's a reason for this behaviour: performance. However, I
think it should be always the second option.

Creating a new reservation is not a usual task, so the best effort
should prevail versus performance at creation time.