Function do_copychunk 

async fn do_copychunk(
    conn: &mut Smb2Conn,
    vfs: Arc<dyn Vfs>,
    req: &IoctlReq,
) -> Result<Vec<u8>, (Status, Vec<u8>)>

Perform a server-side copy: for each chunk read from the source open named by the resume key and write into the target handle. Returns the SRV_COPYCHUNK_RESPONSE body, or (status, body) where the body carries the server limits on a limits violation ([MS-SMB2] §3.3.5.15.6).