net: sched: cls_api: introduce tc_cls_bind_class() helper

All the bind_class callback duplicate the same logic, this patch
introduces tc_cls_bind_class() helper for common usage.

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