tc: add new queue discipline: head drop fifo

tc: add new queue discipline: head drop fifo

This adds the required changes to gain access to
the head drop classfull queuing discipline named
pfifo_head_drop. In difference to pfifo or pfifo_fast
this queuing discipline will drop the first packet
in the case of queue congestion. As a result the queue
contain always the freshest packets.

To replace the current a root queueing discipline
for eth0:
$ tc qdisc replace dev eth0 root pfifo_head_drop

And show statistics:
$ tc -s qdisc show dev eth0

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>