net: sched: simplify the qdisc_leaf code

Except for returning, the var leaf is not
used in the qdisc_leaf(). For simplicity, remove it.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>