act_police: improved accuracy at high rates

Current act_police uses rate table computed by the "tc" userspace
program, which has the following issue:

The rate table has 256 entries to map packet lengths to token (time
units).  With TSO sized packets, the 256 entry granularity leads to
loss/gain of rate, making the token bucket inaccurate.

Thus, instead of relying on rate table, this patch explicitly computes
the time and accounts for packet transmission times with nanosecond
granularity.

This is a followup to 56b765b7
("htb: improved accuracy at high rates").

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>