std: Store flowinfo/scope_id in host byte order

[alexcrichton]

Apparently these aren't supposed to be stored in network byte order, so doing so
ends up causing failures when it would otherwise succeed when stored in the host
byte order.

Closes #32424

[rust-highfive]

r? @aturon

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[aturon]

@bors: r+

[bors]

📌 Commit 88506ce has been approved by aturon

[eddyb]

@bors rollup

[tamird]

Hm, no test?

[alexcrichton]

I would! but I wasn't sure how :(

[tamird]

Couldn't you use the code sample from #32424 verbatim?

use std::net::{SocketAddr, SocketAddrV6, UdpSocket};

fn main() {
    let socket = UdpSocket::bind(&SocketAddr::V6(SocketAddrV6::new("fe80::1".parse().unwrap(), 0xfc00, 0, 2))).unwrap();
}

[alexcrichton]

I tried that locally, yes, but it was failing as the arguments are system specific (talking bout specific interfaces here and there)