CVE-2023-52577 dccp: fix dccp_v4_err()/dccp_v6_err() again

In the Linux kernel, the following vulnerability has been resolved: dccp: fix dccpv4err/dccpv6err again dh-dccphx is the 9th byte offset 8 in "struct dccphdr", not in the "byte 7" as Jann claimed. We need to make sure the ICMP messages are big enough, using more standard ways no more assumptions...