mlxsw: spectrum_qdisc: Promote backlog reduction to mlxsw_sp_qdisc_destroy()

When a qdisc is removed, it is necessary to update the backlog value at its
parent--unless the qdisc is at root position. RED, TBF and FIFO all do
that, each separately. Since all of them need to do this, just promote the
operation directly to mlxsw_sp_qdisc_destroy(), instead of deferring it to
individual destructors. Since FIFO dtor thus becomes trivial, remove it.

Add struct mlxsw_sp_qdisc.parent to point at the parent qdisc. This will be
handy later as deeper structures are offloaded. Use the parent qdisc to
find the chain of parents whose backlog value needs to be updated.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>