Improve accuracy of rate= option

Improve accuracy of rate= option

I noticed that the rate= option is not terribly precise in some cases.
 It gets worse the higher the selected rate.  For example:
$ fio -name=load -size=100g -ioengine=null -runtime=10 -rate=30m
One would expect that to read 300MB (307200KB) at close to 30MB/s
(30720KB/s).  However it writes 315024KB at 31499KB/s.  Further
experimentation shows that even higher rates can show bigger
discrepancies.  At the extreme end...
$ fio -name=load -size=100g -ioengine=null -runtime=10 -rate=500m
One would expect this to write 5000MB at a rate of 500MB/s
(512000KB/s).  However it writes close to double that (9536.8MB) at a
rate of over 953MB/s.  At a rate of 1GB/s and higher, the rate
limiting is effectively ignored.

This patch improves the accuracy of the rate= option across the whole
range of rates, at the cost of being very slightly more
computationally expensive.

Signed-off-by: Jens Axboe <axboe@kernel.dk>