[MINOR] slightly optimize time calculation for rbtree

The new rbtree-based scheduler makes heavy use of tv_cmp2(), and
this function becomes a huge CPU eater. Refine it a little bit in
order to slightly reduce CPU usage.
diff --git a/src/task.c b/src/task.c
index b49bc74..ca262d6 100644
--- a/src/task.c
+++ b/src/task.c
@@ -39,7 +39,7 @@
 	{
 		parent = *p;
 		task = rb_entry(parent, struct task, rb_node);
-		if (tv_cmp2(&task->expire, &newtask->expire) >= 0)
+		if (tv_cmp_ge2(&task->expire, &newtask->expire))
 			p = &(*p)->rb_left;
 		else
 			p = &(*p)->rb_right;
@@ -82,7 +82,7 @@
 		node = rb_prev(&task->rb_node);
 		if (node) {
 			prev = rb_entry(node, struct task, rb_node);
-			if (tv_cmp2(&prev->expire, &task->expire) >= 0) {
+			if (tv_cmp_ge(&prev->expire, &task->expire)) {
 				task_delete(task);
 				task->wq = &wait_queue[0];
 				rb_insert_task_queue(task);
@@ -93,7 +93,7 @@
 		node = rb_next(&task->rb_node);
 		if (node) {
 			next = rb_entry(node, struct task, rb_node);
-			if (tv_cmp2(&task->expire, &next->expire) > 0) {
+			if (tv_cmp_ge(&task->expire, &next->expire)) {
 				task_delete(task);
 				task->wq = &wait_queue[0];
 				rb_insert_task_queue(task);