Fix checksum calculation for IPv6 Segment Routing (SRv6) traffic

The checksum calculation for IPv6 packets is based on a pseudo
header that includes the packet's final Destination Address (DA).

If the IPv6 packet contains a Routing header, the final DA is the
last element of the Routing header [RFC8200].

Currently, tcpdump supports Routing header types 0 and 2.

IPv6 Segment Routing Header (SRH) is a new Routing header type (4).
SRH is defined in draft-ietf-6man-segment-routing-header [1].

This patch fix the checksum calculation for SRv6 packets. It allows
tcpdump to get the final DA value from SRv6 packets.

[1] https://tools.ietf.org/html/draft-ietf-6man-segment-routing-header-17