: Remove backlog accounting from TBF, pass limit to the default inner bfifo qdisc only

when TBF was made classful, backlog accounting was kept inside TBF for reasons I don't understand. When used with SFQ as inner qdisc, TBF can not know the exact backlog once SFQ returned NET_CMIT_CN. An underestimate leads to backlog becoming negative when the queue drains, it stops accepting packets then.

This patch removes backlog accounting from TBF, the limit parameter is just passed to the default inner bfifo, once it is replaced limit is not effective anymore.