Handle IPv4 options

[luqmana]

Today we just swallow any IPv4 options we encounter and never emit any ourselves:

opte/lib/opte/src/engine/ip4.rs

Lines 379 to 384 in 1201f21

	// TODO: actually capture and re-emit ipv4 options.
	// before, they were accidentally *becoming* the ULP.
	// now, we're at least skipping them.
	let remaining_bytes = (hdr_len as usize) - Ipv4HdrRaw::SIZE;
	rdr.seek(remaining_bytes)
	.map_err(|_| Ipv4HdrError::HeaderTruncated { hdr_len })?;

Need to decide if we'll just let them through as-is or on a per-option basis. Will we need to rewrite the contents on ingress/egress?

[FelixMcFelix]

It should be said that there is a bug in swallowing these options properly -- we need to adjust the IHL and total_length back down to baseline. As it stands, these are unchanged and so get fed back out when we generate a fresh IP header:

opte/lib/opte/src/engine/ip4.rs

Lines 546 to 562 in 1201f21

	impl From<&Ipv4Meta> for Ipv4HdrRaw {
	#[inline]
	fn from(meta: &Ipv4Meta) -> Self {
	Ipv4HdrRaw {
	ver_hdr_len: 0x45,
	dscp_ecn: 0x0,
	total_len: meta.total_len.to_be_bytes(),
	ident: meta.ident.to_be_bytes(),
	frag_and_flags: [0x40, 0x0],
	ttl: meta.ttl,
	proto: u8::from(meta.proto),
	csum: meta.csum,
	src: meta.src.bytes(),
	dst: meta.dst.bytes(),
	}
	}
	}

The better fix is just to correctly re-emit them, as I promised we should eventually do...