iowatcher: Calculate ios_in_flight per trace

iowatcher: Calculate ios_in_flight per trace

When multiple trace files are passed, the generated queue depth graph
ends up adding queue depths from previous traces.

iowatcher -t blktrace1 -t blktrace2

Assuming blktrace1 has a queue depth of 8 and blktrace2 has a queue depth
of 32, the resulting graph will display blktrace1 with a queue depth of 8
and blktrace2 with a queue depth of 40 instead of 32.

Replace global ios_in_flight with a per trace ios_in_flight to show
a correct queue depth for a given trace.

Signed-off-by: Igor Pylypiv <ipylypiv@google.com>
Link: https://lore.kernel.org/r/20241007211243.1351143-1-ipylypiv@google.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>