RFR(XXS) 8217794: Improve termination detection in owstTaskTerminator

[zgu at redhat.com]

Hi,

Could I get review for this small enhancement, that may improve
termination dectection?

This is *not* a correctness issue, but latency improvement. Since it
already holds lock, it is worth a quick check for termination
condition.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217794
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8217794/webrev.00/

Test:
  hotspot_gc on Linux x64 (fastdebug and release)
  Submit tests

Thanks,

-Zhengyu