Fix LinCowCell chain drop by stopping on shared links

[droideck]

In both lincowcell and lincowcell_async, the drop loop now uses Arc::get_mut to advance only when uniquely owned. If a link is shared, it stops and leaves the chain intact.

[droideck]

The reason for this PR is that without it, concread crashes under heavy load when the NDN cache is enabled. The crash is triggered in the LinCowCellInner::drop chain during commit, where Arc::try_unwrap runs on a non‑unique link and hits an atomic compare_exchange failure.

So we need to use get_mut instead of try_unwrap here to avoid breaking the pinned chain while still preventing recursive drops.