cfq: fix possible problem with jiffies wraparound

[Corrado Zoccolo <czoccolo@gmail.com>]

The RR service tree is indexed by a key that is relative to current jiffies.
This can cause problems on jiffies wraparound.

The patch fixes it using time_before comparison, and changing
the add_front path to use a relative number, too.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
---
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 576e92d..495b16d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -512,8 +512,11 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                rb_key = cfq_slice_offset(cfqd, cfqq) + jiffies;
                rb_key += cfqq->slice_resid;
                cfqq->slice_resid = 0;
-       } else
-               rb_key = 0;
+       } else {
+               rb_key = -HZ;
+               __cfqq = cfq_rb_first(&cfqd->service_tree);
+               rb_key += __cfqq ? __cfqq->rb_key : jiffies;
+       }
 
        if (!RB_EMPTY_NODE(&cfqq->rb_node)) {
                /*
@@ -547,7 +550,7 @@ static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq,
                        n = &(*p)->rb_left;
                else if (cfq_class_idle(cfqq) > cfq_class_idle(__cfqq))
                        n = &(*p)->rb_right;
-               else if (rb_key < __cfqq->rb_key)
+               else if (time_before(rb_key, __cfqq->rb_key))
                        n = &(*p)->rb_left;
                else
                        n = &(*p)->rb_right;