RFR (XXS): 8204617: ParallelGC parallel reference processing does not set MT degree in reference processor

[Thomas Schatzl]

Hi all,

  can I have reviews for this small patch that properly sets the MT
degree in the parallel gc full gc?

Otherwise some assert to be introduced in 8043575: Dynamically
parallelize reference processing work will fire for any application
using parallel gc that does not use the full amount of available gc
threads (by UseDynamicNumberOfGCThreads).

CR:
https://bugs.openjdk.java.net/browse/JDK-8204617
Webrev:
http://cr.openjdk.java.net/~tschatzl/8204617/webrev/
Testing:
hs-tier1-4,jdk-tier1-3 with +/-ParallelRefProcEnabled

Thanks,
  Thomas