Fix accounting issue with sweep_region_in_plan. (#56910)

Fix accounting issue with sweep_region_in_plan. (#56910)

The free list items that we find while doing sweep_region_in_plan are kept on a per-region free list temporarily and later transferred to the per-generation allocator.

We also need to keep track of the free list space and the free obj space to make the accounting work out - otherwise we have items on the per-generation allocator that are unaccounted for, and the per-generation free obj space may become negative. Both cause us to overestimate fragmentation and thus do unnecessary blocking GCs.