Function decode_set_file_op 

pub fn decode_set_file_op(
    class: u8,
    buf: &[u8],
) -> Result<Option<SetOp>, DecodeSetInfoError>

Decode SET_INFO buffer into a neutral [smb_server_vfs::SetOp] where the operation maps onto one; Ok(None) means accepted-but-ignored (e.g. position), and errors mean an unsupported/malformed class.