[SR-7241] libdispatch: queues getting background prio

Issue Description:

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

goes to

 dispatch_queue_t
  dispatch_get_global_queue(long priority, unsigned long flags)
  {
          if (flags & ~(unsigned long)DISPATCH_QUEUE_OVERCOMMIT) {
                  return DISPATCH_BAD_INPUT;
          }    
          dispatch_qos_t qos = _dispatch_qos_from_queue_priority(priority); <---
  #if !HAVE_PTHREAD_WORKQUEUE_QOS
          if (qos == QOS_CLASS_MAINTENANCE) {
                  qos = DISPATCH_QOS_BACKGROUND;
          } else if (qos == QOS_CLASS_USER_INTERACTIVE) {
                  qos = DISPATCH_QOS_USER_INITIATED;
          }    
  #endif
          if (qos == DISPATCH_QOS_UNSPECIFIED) {
                  return DISPATCH_BAD_INPUT;
          }    
          return _dispatch_get_root_queue(qos, flags & DISPATCH_QUEUE_OVERCOMMIT);
  }
 

goes to
_dispatch_qos_from_queue_priority where DISPATCH_QUEUE_PRIORITY_HIGH mapped to DISPATCH_QOS_USER_INITIATED(5) and DISPATCH_QOS_USER_INITIATED(5) is equal to QOS_CLASS_MAINTENANCE(5) and we are getting queue with wrong prio

Expected Results:
High prio queue

Actual Results:
Background prio queue

Version/Build:
master libdispatch

Start by tracing dispatch_get_global_queue through _dispatch_qos_from_queue_priority on Linux when HAVE_PTHREAD_WORKQUEUE_QOS is 0, focusing on the shown priority and QoS values. Reproduce the DISPATCH_QUEUE_PRIORITY_HIGH call and verify that it produces a high-priority queue rather than a background-priority queue.