Allow eBPF to inspect discontiguous frames

[mtfriesen]

1. Provide the first XDP_BUFFER to eBPF for inspection
2. Set a new XDP flag if there are more buffers (multi-buffer), which aligns to the currently understanding of the Linux analogue.
3. Tests

[saxena-anurag]

I think we may need to implement the below helper functions also to get the non-contiguous data?

1. u64 bpf_xdp_get_buff_len(struct xdp_buff *xdp_md)
2. long bpf_xdp_load_bytes(struct xdp_buff *xdp_md, u32 offset, void *buf, u32 len)
3. long bpf_xdp_store_bytes(struct xdp_buff *xdp_md, u32 offset, void *buf, u32 len)

[mtfriesen]

We can, though I'm fine leaving that for a future feature enhancement. Our customer right now only needs to read the L2-L4 headers, which are practically required to be in the first contiguous buffer.