stat: Re-add output of basic bw information if bw_log is not written

stat: Re-add output of basic bw information if bw_log is not written

Starting with 9e4438f ("stat: don't trust per_unit_log() if log is
NULL") fio no longer provides basic bw information (e.g. min, max,
p_agg, mean, stdev) when write_bw_log is not used. Thus you have to
write a bw_log to obtain these values. (But in this case you can
compute those values yourself.) I think this doesn't make sense.  Not
writing entire bw log shouldn't prevent general sampling and
calculation of bw stats.

This patch reactivates sampling for bw and iops to be able to
calculate basic stats for both independend of writing of entire
{bw,iops}_logs.

Also check for log that is really of interest in __add_samples() it
doesn't make sense to check for bw_log if iops information needs to be
written.

Signed-off-by: Andreas Herrmann <aherrmann@suse.com>
Fixes: 9e4438f ("stat: don't trust per_unit_log() if log is NULL")
Signed-off-by: Jens Axboe <axboe@fb.com>