ip6: fix l4 checksum with hop-by-hop header

L4 checksums for IPv6 should be calculated using a pseudo header that
includes the source/destination addresses, payload length, and payload
protocol.

ip6_tcp_udp_icmp_compute_checksum() was using the payload length and
protocol from the IPv6 header. If there is a hop-by-hop header (or any
other extension header), the payload length used for the pseudo header
should only include the upper layer header and payload and not the
extension header bytes. Same deal with the protocol, the upper layer
next header value should be used instead of the extension header.

Type: fix
Fixes: cb9cadad57

Change-Id: Ifa2c9ad41c0fc4eea674f0671255b637c8e01f71
Signed-off-by: Matthew Smith <mgsmith@netgate.com>