Function ioctl 

pub(crate) async fn ioctl(
    conn: &mut Smb2Conn,
    server: &Arc<ServerShared>,
    tid: u32,
    buf: &[u8],
) -> IoctlReply

IOCTL / FSCTL dispatch ([MS-SMB2] §3.3.5.15). Kept as a free function so the typestate handler and its private FSCTL helpers all live in this module.