Add support for BPF_MAP_TYPE_SOCKMAP and related helper functions.

For details and links to relevant references, see https://blog.cloudflare.com/sockmap-tcp-splicing-of-the-future/

- [ ] Map type: `BPF_MAP_TYPE_SOCKMAP` (see above link)
- [ ] Helper: `bpf_sk_redirect_map` (see https://github.com/iovisor/bpf-docs/blob/master/bpf_helpers.rst)
- [ ] Helper: `bpf_msg_redirect_map` (see https://www.man7.org/linux/man-pages/man7/bpf-helpers.7.html)
- [ ] Helper `bpf_sock_map_update`

Begin with the Cloudflare sockmap article and the linked bpf_helpers.rst and man7.org references to understand the requested map and helpers. Trace the repository's existing map-type and helper-support entry points, then verify completion against all four checklist items: BPF_MAP_TYPE_SOCKMAP, bpf_sk_redirect_map, bpf_msg_redirect_map, and bpf_sock_map_update.

Written by the indexing model from the issue text.