iolog: don't serialize the writing of all logs

iolog: don't serialize the writing of all logs

Serialize based on filename, so that independent logs can be
written at the same time. If the logs are big, this can take
a while to flush to disk. It's silly to have all jobs waiting
on each other, when they could write their own logs independently.

Signed-off-by: Jens Axboe <axboe@fb.com>