net/sched/hfsc: allocate tcf block for hfsc root class

Without this filters cannot be attached.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: 6529eaba

 ("net: sched: introduce tcf block infractructure")
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>