Replace the one use of ibuf_prepend() using a similar ibuf_new() + ibuf_cat()

Replace the one use of ibuf_prepend() using a similar ibuf_new() + ibuf_cat()
method but instead of overwriting ibuf internals replace the buf a level up.
Users of ikev2_msg_send() are not allowed to hold and reuse a pointer to
msg_data (which is another footgun to disarm at some point).
OK tb@