The problem is in ip_push_pending_frames(), which uses:

        if (!df) {
                __ip_select_ident(iph, &rt->u.dst, 0);
        } else {
                iph->id = htons(inet->id++);
        }

instead of ip_select_ident().

Right now I think the code is a nonsense. Most likely, I copied it from
old ip_build_xmit(), where it was really special, we had to decide
whether to generate unique ID when generating the first (well, the last)
fragment.

In ip_push_pending_frames() it does not make sense, it should use plain
ip_select_ident() instead.
Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>