Use the existing `is_posix_policy_prio_valid()` function to
verify the POSIX's priority in the conversion functions.
Changed the `priority` arg of `is_posix_policy_prio_valid` to
`int` since that is the output of `sched_get_priority_min` &
`sched_get_priority_max`.
Signed-off-by: Yong Cong Sin <ycsin@meta.com>