iolog: switch to list based scheme

iolog: switch to list based scheme

The previous iolog implementation had one big log, that we continually
resized when we ran out of space. The resize involved a realloc()
of the data, which could potentially be slow for big logs. This could
introduce long latency outliers for workloads, since fio did not
quiesce IO before resizing the log.

Rewrite the logging code to have a list of logs, and cap each of them
at a reasonable size. This means we never have to realloc data, and
that we allocate smaller chunks instead.

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