bcachefs fixes for 6.10-rc2

- two downgrade fixes
- a couple snapshot deletion and repair fixes, thanks to noradtux for
  finding these and providing the image to debug them
- a couple assert fixes
- convert to folio helper, from Matthew
- some improved error messages
- bit of code reorganization (just moving things around); doing this
  while things are quiet so I'm not rebasing fixes past reorgs
- don't return -EROFS on inconsistency error in recovery, this confuses
  util-linux and has it retry the mount
- fix failure to return error on misaligned dio write; reported as an
  issue with coreutils shred
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