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

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

Patch Review

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

```c
+MODULE_LICENSE("GPL");
+MODULE_IMPORT_NS("DMA_BUF");
+MODULE_IMPORT_NS("DMA_BUF_HEAP");
```

**Issue - No `module_exit` function:**
The patch adds `MODULE_LICENSE` but does not add a `module_exit()` handler. When built as a module, this means the module cannot be unloaded. While this may be acceptable (many kernel modules are not unloadable), it should either:
1. Be explicitly documented/noted in the commit message that the module is intentionally non-unloadable, or
2. Have a `module_exit()` added that calls `dma_heap_remove()` or similar cleanup (if such an API exists).

Currently the existing code already has `module_init(add_cma_heaps)` but no exit. This works for built-in, but for a module it means `rmmod` will fail.

**Issue - `__init` annotations become no-ops for modules:**
Both `__add_cma_heap` and `add_cma_heaps` are marked `__init`:

```c
static int __init __add_cma_heap(struct cma *cma, const char *name)
...
static int __init add_cma_heaps(void)
```

When compiled as a module, `__init` is defined as nothing (it's a no-op for modules). This is safe but means the functions persist in module memory. This is the standard behavior and not a bug, just worth being aware of.

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