Rename EMPTY_TASKQ into taskq_empty

To follow a change in illumos taskq

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12802

lib/libzpool/taskq.c

@@ -363,6 +363,12 @@ taskq_cancel_id(taskq_t *tq, taskqid_t id)
 	return (ENOENT);
 }
 
+boolean_t
+taskq_empty(taskq_t *tq)
+{
+	return (tq->tq_task.tqent_next == &tq->tq_task || tq->tq_active == 0);
+}
+
 void
 system_taskq_init(void)
 {