libnl: add initial OSS-Fuzz integration

[bybel]

Summary

libnl is a C library for working with the Linux Netlink protocol. It is widely used by NetworkManager, systemd, iproute2, and other core Linux networking components, making it a high-value fuzzing target for memory safety and parsing correctness issues.

This PR adds an initial OSS-Fuzz integration with 7 fuzz targets covering all major sub-libraries:

Fuzzer	Library	Coverage
fuzz_msg	libnl-3	nlmsg_parse, nlmsg_validate, nlmsg_find_attr
fuzz_attr	libnl-3	nla_parse, nla_validate, nla_find
fuzz_addr	libnl-3	nl_addr_parse across all address families
fuzz_route	libnl-route-3	rtnl_route_parse (RTM_NEWROUTE)
fuzz_neigh	libnl-route-3	rtnl_neigh_parse (RTM_NEWNEIGH)
fuzz_ct	libnl-nf-3	nfnlmsg_ct_parse (netfilter conntrack)
fuzz_idiag	libnl-idiag-3	idiagnl_msg_parse (inet socket diagnostics)

Testing

Verified locally with python3 infra/helper.py check_build libnl:

INFO: performing bad build checks for fuzz_route
INFO: performing bad build checks for fuzz_idiag
INFO: performing bad build checks for fuzz_msg
INFO: performing bad build checks for fuzz_addr
INFO: performing bad build checks for fuzz_ct
INFO: performing bad build checks for fuzz_neigh
INFO: performing bad build checks for fuzz_attr
Check build passed.

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[ghost]

bybel is integrating a new project:
- Main repo: https://github.com/thom311/libnl
- Criticality score: 0.56589