net_sched: tbf: support of 64bit rates

With psched_ratecfg_precompute(), tbf can deal with 64bit rates.
Add two new attributes so that tc can use them to break the 32bit
limit.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Suggested-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>