[RBTREE] Update hrtimers to use rb_parent() accessor macro.

Also switch it to use the same method of using off-tree nodes as
everyone else now does -- set them to point to themselves.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

include/linux/hrtimer.h

@@ -127,7 +127,7 @@ extern ktime_t hrtimer_get_next_event(void);
 
 static inline int hrtimer_active(const struct hrtimer *timer)
 {
-	return timer->node.rb_parent != HRTIMER_INACTIVE;
+	return rb_parent(&timer->node) != &timer->node;
 }
 
 /* Forward a hrtimer so it expires after now: */