mlxsw: spectrum_qdisc: Extract a common leaf unoffload function

When the RED Qdisc is unoffloaded, it needs to reduce the reported backlog
by the amount that is in the HW, so that only the SW backlog is contained
in the counter. The same thing will need to be done by TBF, and likely any
other leaf Qdisc as well.

Extract a helper mlxsw_sp_qdisc_leaf_unoffload() and call it from
mlxsw_sp_qdisc_red_unoffload().

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>