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

module/os/linux/spl/spl-taskq.c

@@ -1229,6 +1229,13 @@ taskq_destroy(taskq_t *tq)
 }
 EXPORT_SYMBOL(taskq_destroy);
 
+boolean_t
+taskq_empty(taskq_t *tq)
+{
+	return (tq->tq_lowest_id == tq->tq_next_id);
+}
+EXPORT_SYMBOL(taskq_empty);
+
 static unsigned int spl_taskq_kick = 0;
 
 /*