[dhcpv4] Fix buffer end location

[noiz]

Fix payload buffer to end at actual payload end. UDP header length includes the 8 bytes of the header so we do not want the +8 at the end of the buffer.

In our testing on ARM platforms we were hitting the return ErrInvalidOptions error when parsing DHCPv4 options at the end of the payload. It turns out the error was here that the payload buffer was 8 bytes too long so sometimes there would be garbage bytes after the optEnd byte that would cause this error to get returned. With this change we are now seeing the payload buffer size in the Golang code matches the payload buffer size we see in wireshark.

[pmazzini]

Please sign your commit, example:

git commit --amend --no-edit --signoff
git push --force-with-lease origin master

[codecov]

Codecov Report

Merging #447 (3c6a668) into master (63954f9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #447   +/-   ##
=======================================
  Coverage   67.97%   67.97%           
=======================================
  Files          90       90           
  Lines        3766     3766           
=======================================
  Hits         2560     2560           
  Misses       1037     1037           
  Partials      169      169           

Flag	Coverage Δ
integtests	∅ <ø> (∅)
unittests	67.97% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 63954f9...3c6a668. Read the comment docs.

[pmazzini]

seems legit, thanks