Use exclusive threads to optimize work wait queue

When a single work item is dispatched we only need to wake up one
thread to service it.  The current taskq implementation uses
non-exclusive processes in the work wait queue, so every thread in the
queue gets woken up when __taskq_dispatch() calls wake_up().  This
overhead can become non-negligible as the number of threads in the
queue increases, particularly when a large number of short work items
are being dispatched.  For example, this scenario appears to have led
to poor unlink performance in ZFS as described in Issues
openzfs/zfs#458 and openzfs/zfs#258.  To optimize this case, use
add_wait_queue_exclusive() to add threads to the work queue so only
one is woken up at a time.

module/spl/spl-taskq.c

@@ -454,7 +454,7 @@ taskq_thread(void *args)
 
         while (!kthread_should_stop()) {
 
-		add_wait_queue(&tq->tq_work_waitq, &wait);
+		add_wait_queue_exclusive(&tq->tq_work_waitq, &wait);
 		if (list_empty(&tq->tq_pend_list) &&
 		    list_empty(&tq->tq_prio_list)) {
 			spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);