pkt_sched: act_police: Fix a rate estimator test.

A commit c1b56878

 "tc: policing requires
a rate estimator" introduced a test which invalidates previously working
configs, based on examples from iproute2: doc/actions/actions-general.
This is too rigorous: a rate estimator is needed only when police's
"avrate" option is used.

Reported-by: Joao Correia <joaomiguelcorreia@gmail.com>
Diagnosed-by: John Dykstra <john.dykstra1@gmail.com>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>