[PKT_SCHED]: Transform fifo qdisc to use generic queue management interface

The simplicity of the fifo qdisc allows several qdisc operations to be
redirected to the relevant queue management function directly. Saves
a lot of code lines and gives the pfifo a byte based backlog.

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