ipv6: ndisc: fix NULL deref in accept_untracked_na()

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UBUNTU-CVE-2026-64542

In the Linux kernel, the following vulnerability has been resolved: ipv6: ndisc: fix NULL deref in acceptuntrackedna acceptuntrackedna re-fetches the inet6dev with in6devgetdev and dereferences idev-cnf.acceptuntrackedna without a NULL check, even though its only caller ndiscrecvna already fetche...