checklocks: enforce neighbor state ownership

Neighbor completion schedules dequeue while holding the entry mutex.
Since 931f9709f00, the callback reads the link address at execution time
without that mutex. Use the existing completion result captured under
the lock, keeping dequeue asynchronous to preserve queue/neighbor lock
ordering.

Restore the missing cache and entry guards from #7058 with the current
lockdep mutexes. Take read-locked snapshots in existing cache assertions
so the same ownership contract is enforced in tests.

Assisted-by: Codex