Claude review: lib: test_hmm: Implement a device release method

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Good fix for the warning.** The release method is correctly wired up.

**Reordering of dmirror_allocate_chunk before cdev_device_add is a nice improvement:**

```c
/* Build a list of free ZONE_DEVICE struct pages */
ret = dmirror_allocate_chunk(mdevice, NULL, false);
if (ret)
    goto put_device;

ret = cdev_device_add(&mdevice->cdevice, &mdevice->device);
if (ret)
    goto put_device;
```

Previously, `dmirror_allocate_chunk` was called *after* `cdev_device_add`, meaning if allocation failed, `cdev_device_del()` was never called — the device was added but not properly removed. The new ordering is better: allocate resources first, only then expose the device. The `put_device` error path now correctly triggers `dmirror_device_release()` which calls `dmirror_device_remove_chunks()`, and `dmirror_device_remove_chunks()` safely handles the case where chunks haven't been allocated (the `if (mdevice->devmem_chunks)` guard).

**One subtle concern:** After the reorder, if `cdev_device_add()` fails, `put_device()` triggers `dmirror_device_release()` → `dmirror_device_remove_chunks()`. The `remove_chunks` function calls `dmirror_device_evict_chunk()` on each chunk. Since the device was never made visible (no `cdev_device_add`), no user could have migrated pages to the device memory, so the eviction should find no migrated pages and be harmless. This is safe.

**Overall:** Clean and correct. The error-handling improvement is a nice side-fix.

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Generated by Claude Code Patch Reviewer