net/sched/sch_hfsc.c: initialize parent's cl_cfmin properly in init_vf()

This patch fixes init_vf() function, so on each new backlog period parent's
cl_cfmin is properly updated (including further propgation towards the root),
even if the activated leaf has no upperlimit curve defined.

Signed-off-by: Michal Soltys <soltys@ziu.info>
Signed-off-by: David S. Miller <davem@davemloft.net>