My previous downstream changes introduced a regression. Use instead
upstream tested change, which makes smaller change and does not create
new regression in relay code.
Resolves: rhbz#2126586
Long-term pointer to beginning of message does not work well. I case
outpacket is reallocated in any new_opt6() section, original outmsgtypep
pointer becomes invalid. Instead of using that pointer use dedicated
function, which will change just the first byte of the message.
This makes sure correct beginning of packet is always used.
Resolves: CVE-2022-0934