Modify and refactor smn_btrfs.sh

Currently, the [btrfs_fix](https://github.com/SaveMyNode/savemynode/blob/main/scripts/smn_btrfs.sh) script is not suitable for our needs.

Refactor and modify everything necessary (even if it is from scratch) to ensure that all our needs for SaveMyNode is satisfied

Also ensure finally that it follows [expectations](https://github.com/SaveMyNode/savemynode/blob/main/product/mvp.md)