Make return value type of fio_getaffinity() consistent

Make return value type of fio_getaffinity() consistent

Return type of fio_getaffinity() isn't consistent among supported OS.
Windows and DragonFlyBSD return void while FreeBSD version is int.
The default version for those that don't support is do{}while(0).

Linux version is a macro for sched_getaffinity(2) which returns 0
on success and -1 otherwise, so others should basically follow that.

Note that I haven't compiled this on Windows, but it shouldn't fail.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>