selftests/bpf: Add LWT encap tests for skb metadata

selftests/bpf: Add LWT encap tests for skb metadata

Test that an LWT encapsulation does not silently corrupt XDP metadata
sitting in the skb headroom. Exercise all three LWT dispatch paths:

- BPF LWT xmit prog reserves headroom on the LWT .xmit redirect,
- mpls pushes an MPLS label on the LWT .xmit redirect,
- seg6 in encap mode runs on the LWT .input redirect,
- ioam6 encap inserts an IOAM Hop-by-Hop option on LWT .output redirect.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://patch.msgid.link/20260619-bpf-lwt-drop-skb-metadata-v3-2-71d6a33ab76b@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>