DNS grain doesn't work

[lberezy]

I think this is related to foniod/redbpf#102

I can get it to "work" by replacing the header test with the following.

if transport.source() != 53 {
        return Ok(XdpAction::Pass);
    }

Explicitly testing the length of the header before access is not enough it seems.
This doesn't work.

    let header = data.slice(12)?;
    if header.len() == 12 {
        if header[2] >> 3 & 0xF != 0u8 {
            return Ok(XdpAction::Pass);
        }
    }