6915005: G1: Hang in PtrQueueSet::completed_buffers_list_length with gcl001 Summary: When enqueuing a completed PtrQueue buffer, cache a local pointer to the buffer and clear the field in the PtrQueue prior to unlocking the mutex referenced by the _lock field and pass the cached local value to the enqueuing routine. This will prevent the same completed buffer being enqueued multiple times, which causes the hang. Reviewed-by: ysr