Revert "Mark all public struct/enum as non_exhaustive"

[hch12907]

This reverts commit 53a4c4e.

While the idea of having forward compatibility was sound, the commit effectively disallowed any Netlink message to be created:

    let mut packet = NetlinkMessage {
        header: NetlinkHeader {
            sequence_number: 1,
            flags: NLM_F_DUMP | NLM_F_REQUEST,
            ..Default::default()
        },
        payload: RtnlMessage::GetLink(LinkMessage::default()).into(),
    };

error[E0639]: cannot create non-exhaustive struct using struct expression

This is just silly.

[cathay4t]

Please use NetlinkMessage::new. For example: https://github.com/rust-netlink/netlink-packet-route/blob/main/src/tests.rs#L31