rebalance_work accounting overflow/underflow

[g2p]

Noticed this from in-kernel fsck (mount -o verbose,fsck,fix_errors):

bcachefs (fff…): check_allocations...
accounting mismatch for rebalance_work : got 18446744073709538648 should be 207994600, fixing
accounting mismatch for compression lz4: got 1374452 173195248 26964280 should be 1309912 165572536 23486472, fixing
accounting mismatch for compression zstd: got 39845537 4984839376 726231984 should be 39845509 4937226704 719136600, fixing
accounting mismatch for compression incompressible: got 202911546 16270674800 16270674800 should be 202900684 15899707176 15899707176, fixing
done

18446744073709538648 is -12968 cast as u64

[koverstreet]

rebalance_work had an accounting bug that was just fixed - but there must be a bug with compression accounting