net: sched: ensure n arg not empty before call bind_class

All bind_class callbacks are directly returned when n arg is empty.
Therefore, bind_class is invoked only when n arg is not empty.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>