Extract IPv6 payload with the Jumbo option correctly

[evverx]

It's more of a bug report at this point probably but it already contains one bug fix and a few tests reproducing the issue.

I ran into this while I was looking for packs that don't match their unpacks.

[codecov[bot]]

Codecov Report

Merging #3906 (ae95049) into master (669506b) will decrease coverage by 27.23%. The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master    #3906       +/-   ##
===========================================
- Coverage   86.93%   59.71%   -27.23%     
===========================================
  Files         310      285       -25     
  Lines       70599    65592     -5007     
===========================================
- Hits        61378    39169    -22209     
- Misses       9221    26423    +17202     

Impacted Files	Coverage Δ
scapy/layers/inet6.py	40.83% <0.00%> (-47.48%)	:arrow_down:
scapy/contrib/igmp.py	27.69% <0.00%> (-72.31%)	:arrow_down:
scapy/layers/tls/record.py	20.81% <0.00%> (-71.09%)	:arrow_down:
scapy/layers/tls/cert.py	17.56% <0.00%> (-70.61%)	:arrow_down:
scapy/layers/tls/record_sslv2.py	16.75% <0.00%> (-69.28%)	:arrow_down:
scapy/route6.py	19.10% <0.00%> (-69.11%)	:arrow_down:
scapy/route.py	21.53% <0.00%> (-68.47%)	:arrow_down:
scapy/layers/tls/automaton.py	15.78% <0.00%> (-67.67%)	:arrow_down:
scapy/layers/sixlowpan.py	19.68% <0.00%> (-66.90%)	:arrow_down:
scapy/layers/tls/crypto/hkdf.py	35.55% <0.00%> (-64.45%)	:arrow_down:
... and 215 more

[gpotter2]

Interesting PR, what's blocking?

[evverx]

I haven't addressed the TODO parts yet. I put this PR on hold because there are some other issues I wanted to fix first. (They are minor though but they prevent my fuzz target from covering the core part of scapy). I'm slowly working on them. Plus https://github.com/secdev/scapy/pull/3709#issuecomment-1620438888 confirmed that the tests there can safely be changed so I'll probably try to resurrect that PR first. All in all this PR is on the backburner but I'm planning to get back to it eventually.