For `G1BarrierSet::_satb_mark_queue_buffer_allocator`, in remark, after deactivation.

Perhaps the underlying free list should free most or all, to release memory while not doing concurrent GC. There’s little point in carrying them around while concurrent marking is not happening.

The removed `reduce_free_list` in JDK-8286387 might provide much inspiration, even if the code can't be used verbatim.