Fix tenuring threshold reported in GCYoungGarbageCollection event

[Carsten Varming]

Dear Hotspot-gc-dev members,

I noticed that the GCYoungGarbageCollection trace event reports the
tenuring threshold of the next gc, i.e., not the gc with the gcId in the
event, when using ParellelGC or ParNew (G1 seems to be correct). It looks
like the variable _tenuring_threshold is adjusted for the next gc before
the event is fired.

I attached a small patch against the current jdk9 repo that fixes the issue.

Let me know if this is the wrong email list for this type of change.

Carsten
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