tc: pie: add dq_rate_estimator option

tc: pie: add dq_rate_estimator option

PIE now uses per packet timestamps to calculate queuing
delay. The average dequeue rate based queue delay
calculation is now made optional. This patch adds the option
to enable or disable the use of Little's law to calculate
queuing delay.

Signed-off-by: Gautam Ramakrishnan <gautamramk@gmail.com>
Signed-off-by: Leslie Monis <lesliemonis@gmail.com>
Signed-off-by: Mohit P. Tahiliani <tahiliani@nitk.edu.in>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>