selftests/net: expand cmsg_ipv6.sh with ipv4

selftests/net: expand cmsg_ipv6.sh with ipv4

Expand IPV6_TCLASS to also cover IP_TOS.
Expand IPV6_HOPLIMIT to also cover IP_TTL.

Expand csmg_sender.c to allow setting IPv4 setsockopts.
Also rename struct v6 to cmsg to match its expanded scope.
Don't bother updating all occurrences of tclass and hoplimit.

Rename cmsg_ipv6.sh to cmsg_ip.sh to match the expanded scope.

Be careful around the subtle API difference between TCLASS and TOS.
IP_TOS includes ECN bits. Add a test to verify that these are masked
when making routing decisions.

Diff is more concise with --word-diff

Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250225022431.2083926-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>