change fio_set_odirect() prototype not to use int fd

change fio_set_odirect() prototype not to use int fd

This function should take struct fio_file*, since the idea of integer
fd in unix likes isn't necessarily usable on other platforms (Windows).

In fact, other fd related functions under os/ don't take int fd for
portability even if some of them work fine with int fd at the moment.

(OS headers basically (should)have no dependencies on fio functions
and structures, but struct fio_file is the only exception.)

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