Claude review: dma-buf: heaps: system: Turn the heap into a module

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

Patch Review

```c
-	bool "DMA-BUF System Heap"
+	tristate "DMA-BUF System Heap"
```

```c
+MODULE_DESCRIPTION("DMA-BUF System Heap");
+MODULE_LICENSE("GPL");
+MODULE_IMPORT_NS("DMA_BUF");
+MODULE_IMPORT_NS("DMA_BUF_HEAP");
```

**Same issue - No `module_exit` function:** Same concern as patch 6. The system heap also has no `module_exit()`, making it non-unloadable as a module.

**Minor observation:** The system heap previously had no `MODULE_DESCRIPTION`, so this patch adds it. That's good. The CMA heap already had one (added to existing code), so patch 6 only adds `MODULE_LICENSE` and the `MODULE_IMPORT_NS` lines, which is correct.

**No other issues.** The system heap is simpler (no CMA dependency inversion needed) so this patch is straightforward.

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**Summary of issues:**
1. **Missing `module_exit()`** in both CMA and system heaps (patches 6 and 7). Without an exit function, these modules cannot be unloaded. This should be either intentional (and documented) or addressed.
2. **`rmem_cma_areas` lifetime** (patch 1): Correctly not `__initdata` for module support, but the permanent memory cost could be mentioned in the commit message.
3. **Series ordering in mbox**: The mbox has patches arriving out of order (3, 2, 5, 4 based on lore delivery), but the actual patch numbering in subjects is correct. This is just a mail delivery artifact, not an issue.

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