stat: eliminate extra log samples

stat: eliminate extra log samples

b2a432bfbb6d inadvertently added extra log samples.

$ ./fio-canonical/fio --name=test --time_based --runtime=10s --write_lat_log=fio-07-b2a432 --log_avg_msec=1000 --size=1G --rw=rw
test: (g=0): rw=rw, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=psync, iodepth=1
fio-3.17-93-gb2a4
Starting 1 process
...
$ cat fio-07-b2a432_clat.1.log
1000, 5851, 0, 0, 0
1000, 2551, 1, 0, 0
1000, 5028, 1, 0, 0
2000, 4175, 0, 0, 0
2000, 3214, 1, 0, 0
2000, 60619, 0, 0, 0
...

There should only be two lines at each timestamp (one for reads, one for
writes), but the first two timestamps have three lines each.

The cause is an inadvertent change in stat.c:add_log_sample() of
__add_stat_to_log to _add_stat_to_log. Reverting to the two-underscore
version resolves this issue.

Fixes: https://github.com/axboe/fio/issues/947
Fixes: b2a432bfbb6d ("Per-command priority: Priority logging and libaio/io_uring cmdprio_percentage")
Signed-off-by: Vincent Fu <vincent.fu@wdc.com>