Bcachefs: Better handling of checksum errors/bitrot

> - Extents need a poison bit: "reads return errors, even though it now has a good checksum" - this was added in a separate patch queued up for 6.15.

To me, it sounds like saving the old hash could be useful. That way a human can look at the now-corrupt extent, test flipping that one odd-looking bit, and confirm that the extent now matches the old hash.

So poison bit => failing_hash: Option<Hash>