sch_hfsc: report backlog information

Add backlog (byte count) information in hfsc classes and qdisc, so that
"tc -s" can report it to user, instead of 0 values :

qdisc hfsc 1: root refcnt 6 default 20
 Sent 45141660 bytes 30545 pkt (dropped 0, overlimits 91751 requeues 0)
 rate 1492Kbit 126pps backlog 103226b 74p requeues 0
...
class hfsc 1:20 parent 1:1 leaf 1201: rt m1 0bit d 0us m2 400000bit ls m1 0bit d 0us m2 200000bit
 Sent 49534912 bytes 33519 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 81822b 56p requeues 0
 period 23 work 49451576 bytes rtwork 13277552 bytes level 0
...

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: John A. Sullivan III <jsullivan@opensourcedevel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>