net: sched: red: don't reset the backlog on every stat dump

net: sched: red: don't reset the backlog on every stat dump

Commit bfb8c23aacb4 ("sch_red: report backlog information") copied
child's backlog into RED's backlog.  Back then RED did not maintain
its own backlog counts.  This has changed after commit 329ca48484a3
("net_sched: update hierarchical backlog too") and commit 344a46181fff
("sch_red: update backlog as well").  Copying is no longer necessary.

Tested:

$ tc -s qdisc show dev veth0
qdisc red 1: root refcnt 2 limit 400000b min 30000b max 30000b ecn
 Sent 20942 bytes 221 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 1260b 14p requeues 14
  marked 0 early 0 pdrop 0 other 0
qdisc tbf 2: parent 1: rate 1Kbit burst 15000b lat 3585.0s
 Sent 20942 bytes 221 pkt (dropped 0, overlimits 138 requeues 0)
 backlog 1260b 14p requeues 14

Recently RED offload was added.  We need to make sure drivers don't
depend on resetting the stats.  This means backlog should be treated
like any other statistic:

  total_stat = new_hw_stat - prev_hw_stat;

Adjust mlxsw.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Nogah Frankel <nogahf@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>