net_sched: sch_sfq: add backlog info in sfq_dump_class_stats()

We currently return for each active SFQ slot the number of packets in
queue. We can also give number of bytes accounted for these packets.

tc -s class show dev ifb0

Before patch :

class sfq 11:3d9 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 0b 3p requeues 0
 allot 1266

After patch :

class sfq 11:3e4 parent 11:
 (dropped 0, overlimits 0 requeues 0)
 backlog 4380b 3p requeues 0
 allot 1212

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