fix(eBPF): explicitly attach in SKB mode (#8628)

It appears that the gVNIC driver in Google Cloud doesn't give us enough
headroom to use `bpf_xdp_adjust_head` with a delta of 4 bytes.
Currently, we are loading the XDP program with default flags. By loading
it explicitly in SKB mode, we should be able to bypass these driver
limitations at the expense of some performance (which should still be
better than userspace!).

Related:
https://github.com/GoogleCloudPlatform/compute-virtual-ethernet-linux/issues/70

rust/relay/server/src/ebpf/linux.rs

@@ -35,7 +35,7 @@ impl Program {
             .try_into()?;
         program.load().context("Failed to load program")?;
         program
-            .attach(interface, XdpFlags::default())
+            .attach(interface, XdpFlags::SKB_MODE)
             .with_context(|| format!("Failed to attached to interface {interface}"))?;
 
         let mut stats = AsyncPerfEventArray::try_from(