Per-port counter cache used by Qdiscs is updated periodically, unless the
port is down. The fact that the cache is not updated for down ports is no
problem for most counters, which are relative in nature. However, backlog
is absolute in nature, and if there is a non-zero value in the cache around
the time that the port goes down, that value just stays there. This value
then leaks to offloaded Qdiscs that report non-zero backlog even if
there (obviously) is no traffic.

The HW does not keep backlog of a downed port, so do likewise: as the port
goes down, wipe the backlog value from xstats.

Fixes: 075ab8ad ("mlxsw: spectrum: Collect tclass related stats periodically")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>