stat: use enum fio_ddir consistently

stat: use enum fio_ddir consistently

Most functions in stat.c uses enum fio_ddir dir both as a parameter
and as a local variable in functions.

int ddir is used in a very few places.

Convert the int ddir uses to enum fio_ddir dir so that the code is
consistent.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220203192814.18552-11-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>