[PKT_SCHED]: Blackhole queueing discipline

Useful in combination with classful qdiscs to drop or
temporary disable certain flows, e.g. one could block
specific ds flows with dsmark.

Unlike the noop qdisc it can be controlled by the user and
statistic accounting is done.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>