tc : SFB flow scheduler

tc : SFB flow scheduler

Supports SFB qdisc (included in linux-2.6.39)

1) Setup phase : accept non default parameters

2) dump information

qdisc sfb 11: parent 1:11 limit 1 max 25 target 20
  increment 0.00050 decrement 0.00005 penalty rate 10 burst 20 (600000ms 60000ms)
 Sent 47991616 bytes 521648 pkt (dropped 549245, overlimits 549245 requeues 0)
 rate 7193Kbit 9774pps backlog 0b 0p requeues 0
  earlydrop 0 penaltydrop 0 bucketdrop 0 queuedrop 549245 childdrop 0 marked 0
  maxqlen 0 maxprob 0.00000 avgprob 0.00000

Signed-off-by: Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>