blktrace: add option to iterate over a trace multiple times

blktrace: add option to iterate over a trace multiple times

Scaling a particular trace may result in different runtimes among the
merging traces. By knowing the approximate length of each trace as a
user, the overall runtime of each can be tuned to line up by letting
certain traces loop multiple times.

First, the last timestamp of a trace is recorded at the end of the first
iteration to denote the length of a trace. This value is then used to
offset subsequent iterations of a trace.

Next, the "--merge_blktrace_iters" option is introduced to let the user
specify the number of times to loop over each specific trace. This is
done by passing a comma separated list that index-wise pairs with the
passed files in "--read_iolog". Iteration counts are introduced as well
as keeping track of the length of each trace.

In an example, given two traces, A and B, each 60s long. If we want to
see the impact of trace A issuing IOs twice as fast, the
--merge_blktrace_scalars="50:100" can be set and then
--merge_blktrace_iters="2:1". This runs trace A at 2x the speed twice for
approximately the same runtime as a single run of trace B.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>