Fix IKEv2 critical bit set

[dmaciejak]

According to the RFC, "Critical (1 bit) - MUST be set to zero if the sender wants the recipient to skip this payload if it does not understand the payload type code in the Next Payload field of the previous payload. MUST be set to one if the sender wants the recipient to reject this entire message". The fields are laid out in big endian order, it means to set the bit to critical we have to set the MSB to 1.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.72%. Comparing base (3cbb62e) to head (9cb9d02).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4647      +/-   ##
==========================================
- Coverage   81.64%   78.72%   -2.92%     
==========================================
  Files         359      334      -25     
  Lines       86237    81038    -5199     
==========================================
- Hits        70405    63801    -6604     
- Misses      15832    17237    +1405     

Files with missing lines	Coverage Δ
scapy/contrib/ikev2.py	95.28% <ø> (-0.03%)	⬇️

... and 286 files with indirect coverage changes

[gpotter2]

Thanks for the PR. Tested this and it seems correct

[gpotter2]

LGTM. Added a unit test