When users have a non-root HomeDir (e.g., '/sftp/user'), their SFTP
operations should be relative to that directory. Previously, when a
user uploaded to '/' via SFTP, the path was not translated to their
home directory, causing 'permission denied for / for permission write'.
This fix adds a toAbsolutePath() method that implements chroot-like
behavior where the user's HomeDir becomes their root. All file and
directory operations now translate paths through this method.
Example: User with HomeDir='/sftp/user' uploading to '/' now correctly
maps to '/sftp/user'.
Fixes: https://github.com/seaweedfs/seaweedfs/issues/7470