Function register_ring_call 

pub fn register_ring_call(
    caller: TaskId,
    waiting_on: TaskId,
    ring_id: u64,
    user_data: u64,
    reply_buf: u64,
)

Register a pending ring-based IpcCall for caller.

When the server calls deliver_reply(caller, msg), the kernel will:

• Copy msg into the caller’s buffer at reply_buf.
• Push a CQE to the caller’s ring with the given user_data.