Add ioprio_set() support for DragonFlyBSD
It basically has the same interface as Linux kernel's ioprio_set(2),
but needed to workaround a compile issue by defining it as a macro
as mentioned in a comment (it can be called without syscall(NR, ...)).
This commit is based on the last two commits which made I/O priority
option more generic for non Linux environment.
# uname
DragonFly
# fio --cmdhelp | grep prioclass -B1
prio : Set job IO priority value
prioclass : Your platform does not support IO priority classes
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>