[Cake] Dropping last_len from stats collection & display.

[Kevin Darbyshire-Bryant]

Gentlemen,

Two pull requests https://github.com/dtaht/sch_cake/pull/26  & 
https://github.com/dtaht/tc-adv/pull/11

These drop the collection of last packet's length for stats purposes.

I've not dropped maxlen as I have personally found that *incredibly* 
useful in working out what overheads linux has already accounted for on 
a variety of connections.  It is also a useful indicator of GSO/GRO 
activity.

Not updating another variable (memory write) for each packet may help 
cake's CPU usage a little.

Done in a backwards compatible way for tc.

Please ACK/NACK as you see fit.

Kevin