6983311: G1: LoopTest hangs when run with -XX:+ExplicitInvokesConcurrent Summary: Clear the concurrent marking "in progress" flag while the FullGCCount_lock is held. This avoids a race that can cause back to back System.gc() calls, when ExplicitGCInvokesConcurrent is enabled, to fail to initiate a marking cycle causing the requesting thread to hang. Reviewed-by: tonyp, ysr

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