net_sched: gen_estimator: account for timer drifts

Under heavy stress, timer used in estimators tend to slowly be delayed
by a few jiffies, leading to inaccuracies.

Lets remember what was the last scheduled jiffies so that we get more
precise estimations, without having to add a multiply/divide in the loop
to account for the drifts.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>