ipv6: save dontfrag in cork

When spanning datagram construction over multiple send calls using
MSG_MORE, per datagram settings are configured on the first send.

That is when ip(6)_setup_cork stores these settings for subsequent use
in __ip(6)_append_data and others.

The only flag that escaped this was dontfrag. As a result, a datagram
could be constructed with df=0 on the first sendmsg, but df=1 on a
next. Which is what cmsg_ip.sh does in an upcoming MSG_MORE test in
the "diff" scenario.

Changing datagram conditions in the middle of constructing an skb
makes this already complex code path even more convoluted. It is here
unintentional. Bring this flag in line with expected sockopt/cmsg
behavior.

And stop passing ipc6 to __ip6_append_data, to avoid such issues
in the future. This is already the case for __ip_append_data.

inet6_cork had a 6 byte hole, so the 1B flag has no impact.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250307033620.411611-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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