net_sched: sfq: always randomize hash perturbation

SFQ q->perturbation is used in sfq_hash() as an input to Jenkins hash.

We currently randomize this 32bit value only if a perturbation timer is
setup.

Its much better to always initialize it to defeat attackers, or else
they can predict very well what kind of packets they have to forge to
hit a particular flow.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>