Use 0 instead of DDIR_READ to iterate from 0 to DDIR_RWDIR_CNT

I think this is better since we want to count up DDIR_RWDIR_CNT,
but not specifically (count - something) even if something is 0,
and that's basically what DDIR_RWDIR_CNT is there for to equal
SYNC=3 after READ/WRITE/TRIM in io_ddir.h.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>