6lowpan: fix NHC entry use-after-free on error path

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CVE-2026-64452

In the Linux kernel, the following vulnerability has been resolved: 6lowpan: fix NHC entry use-after-free on error path lowpannhcdouncompression looks up an NHC descriptor while holding lowpannhclock. If the descriptor has no uncompress callback, the error path drops the lock before printing...