Make I/O priority option generic for non-Linux environment [1/2]

Make I/O priority option generic for non-Linux environment [1/2]

There is a kernel that supports I/O priority with a syscall similar
to Linux (e.g. DragonFlyBSD's ioprio_set(2) which apparently seems
to have been inspired by ioprio_set(2) in Linux kernel), however
min/max value of the priority may differ, so use OS specific macros
instead of hardcoded values that are designed to work on Linux.

This commit adds IOPRIO_MIN|MAX_PRIO and IOPRIO_MIN|MAX_PRIO_CLASS
to Linux (incl Android), but no functional changes.

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