sch_tbf: report backlog information

Provide child qdisc backlog (byte count) information so that "tc -s
qdisc" can report it to user.

qdisc netem 30: root refcnt 18 limit 1000 delay 20.0ms  10.0ms
 Sent 948517 bytes 898 pkt (dropped 0, overlimits 0 requeues 1)
 rate 175056bit 16pps backlog 114b 1p requeues 1
qdisc tbf 40: parent 30: rate 256000bit burst 20Kb/8 mpu 0b lat 0us
 Sent 948517 bytes 898 pkt (dropped 15, overlimits 611 requeues 0)
 backlog 18168b 12p requeues 0

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>