fsperf: Add per-work (workqueue item) scheduler stats

Now, we can't know detail of what kworker is doing. Because kworker thread
might be running several work items while scheduled.

So, this separates schedule time per-work.

    $ fsperf.pl report -w 1

With above option (-w), user can specify work item like pid.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>