pkt_sched: gen_estimator: Fix signed integers right-shifts.

Right-shifts of signed integers are implementation-defined so unportable.

With feedback from: Eric Dumazet <dada1@cosmosbay.com>

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>