net_sched: remove a bogus warning in hfsc

In update_vf():

  cftree_remove(cl);
  update_cfmin(cl->cl_parent);

the cl_cfmin of cl->cl_parent is intentionally updated to 0
when that parent only has one child. And if this parent is
root qdisc, we could end up, in hfsc_schedule_watchdog(),
that we can't decide the next schedule time for qdisc watchdog.
But it seems safe that we can just skip it, as this watchdog is
not always scheduled anyway.

Thanks to Marco for testing all the cases, nothing is broken.

Reported-by: Marco Berizzi <pupilla@libero.it>
Tested-by: Marco Berizzi <pupilla@libero.it>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>