selftests/net: expand cmsg_ip with MSG_MORE
UDP send with MSG_MORE takes a slightly different path than the
lockless fast path.
For completeness, add coverage to this case too.
Pass MSG_MORE on the initial sendmsg, then follow up with a zero byte
write to unplug the cork.
Unrelated: also add two missing endlines in usage().
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250307033620.411611-4-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>