BUG/MINOR: net_helper: fix IPv6 header length processing

BUG/MINOR: net_helper: fix IPv6 header length processing

The IPv6 header contains a payload length that excludes the 40 bytes of
IPv6 packet header, which differs from IPv4's total length which includes
it. As a result, the parser was wrong and would only see the IP part and
not the TCP one unless sufficient options were present tocover it.

This issue came in 3.4-dev2 with recent commit e88e03a6e4 ("MINOR:
net_helper: add ip.fp() to build a simplified fingerprint of a SYN"),
so no backport is needed.