RFR: 8196865: G1 log for active workers is wrong

[Leo Korinth]

Hi,

Updating a local variable after recalculating number of active workers 
(fixes bad logging). I need a sponsor for the change.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8196865

Webrev:
http://cr.openjdk.java.net/~lkorinth/8196865/00/

Testing:
- hs-tier1, hs-tier2 (on linux-x64)

Also, manual testing:

Before:
[0.500s][trace][gc,task] GC(0) GC Thread: using 1 out of 23 workers
[0.500s][info ][gc,task] GC(0) Using 6 workers of 23 for evacuation

After:
[0.511s][trace][gc,task] GC(0) GC Thread: using 1 out of 23 workers
[0.511s][info ][gc,task] GC(0) Using 1 workers of 23 for evacuation

Thanks,
Leo