selftests: net: csum: Clean up recv_verify_packet_ipv6

Rename ip_len to payload_len since the length in this case refers only
to the payload, and not the entire IP packet like for IPv4. While we're
at it, just use the variable directly when calling
recv_verify_packet_udp/tcp.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240930162935.980712-1-sean.anderson@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>