bcachefs fixes for 6.9

- Various syzbot fixes; mainly small gaps in validation
- Fix an integer overflow in fiemap() which was preventing filefrag from
  returning the full list of extents
- Fix a refcounting bug on the device refcount, turned up by new
  assertions in the development branch
- Fix a device removal/readd bug; write_super() was repeatedly dropping
  and retaking bch_dev->io_ref references
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