selftests/net: prepare cmsg_ipv6.sh for ipv4

selftests/net: prepare cmsg_ipv6.sh for ipv4

Move IPV6_TCLASS and IPV6_HOPLIMIT into loops, to be able to use them
for IP_TOS and IP_TTL in a follow-on patch.

Indentation in this file is a mix of four spaces and tabs for double
indents. To minimize code churn, maintain that pattern.

Very small diff if viewing with -w.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250225022431.2083926-2-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>