rate: use constant to avoid repeated definitions

Change-Id: I344cacbad7208d56c9760c890c436f68b326b865
GitHub-Last-Rev: e776f693031b99d348da756bd91959b23c1fa199
GitHub-Pull-Request: golang/time#23
Reviewed-on: https://go-review.googlesource.com/c/time/+/431558
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
diff --git a/rate/rate.go b/rate/rate.go
index 12e813c..79a6bb6 100644
--- a/rate/rate.go
+++ b/rate/rate.go
@@ -125,7 +125,7 @@
 }
 
 // InfDuration is the duration returned by Delay when a Reservation is not OK.
-const InfDuration = time.Duration(1<<63 - 1)
+const InfDuration = time.Duration(math.MaxInt64)
 
 // DelayFrom returns the duration for which the reservation holder must wait
 // before taking the reserved action.  Zero duration means act immediately.