Request for approval for 6526682: JConsole shows negative CPU Usage

[Nils Loodin]

Backport of simple fix to not show negative CPU utilization.

Bug: https://jbs.oracle.com/bugs/browse/JDK-6526682

Webrev:
http://cr.openjdk.java.net/~nloodin/6526682/webrev.00/

Review thread:
http://mail.openjdk.java.net/pipermail/serviceability-dev/2013-May/009976.html

Checkin in JDK8:
http://hg.openjdk.java.net/jdk8/tl-gate/jdk/rev/33d1376bf725

/Nils Loodin