net_sched: htb: do not setup default rate estimators
With a thousand htb classes, est_timer() spends ~5 million cpu cycles
and throws out cpu cache, because each htb class has a default
rate estimator (est 4sec 16sec).
Most users do not use default rate estimators, so switch htb
to not setup ones.
Add a module parameter (htb_rate_est) so that users relying
on this default rate estimator can revert the behavior.