iolog: make write_*_log prefix optional

iolog: make write_*_log prefix optional

Providing write_bw_log=jobname for every job in a jobfile gets pretty
tedious. This way, we can just do, e.g.,

[global]
write_bw_log

and all jobs will be logged. The documentation seems to suggest that
this is the intended behavior, and it looks like that used to be the
case.

Fixes: 203160d52b86 ("options: get rid of more .cb option usage we don't need")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>