'uq_inherited_pri' contains the current priority inherited from Priority
Protection mutexes. If -1 is passed through m_ceilings[1], meaning that
there are no such mutexes held anymore, this must be reflected into it
by setting it to PRI_MAX, regardless of whether the thread has privilege
to set realtime priorities (PRI_MAX is also obviously not a realtime
priority level). By contrast, it shall not be updated and the computed
'new_inherited_pri' shall stay unused if the thread doesn't have the
ability to set a realtime priority, possibly keeping an older such
priority acquired previously.