Claude review: dma: contiguous: Turn heap registration logic around

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

Patch Review

This is the key architectural change. It moves the CMA reserved-region tracking from `cma_heap.c` into `kernel/dma/contiguous.c` and provides an iterator `dma_contiguous_get_reserved_region()` for the heap to use.

**Positive:**
- Clean inversion of the dependency. The old `dma_heap_cma_register_heap()` called from `contiguous.c` into heap code; now the heap calls into `contiguous.c`. This correctly eliminates the built-in-to-module dependency.
- Proper `!CONFIG_DMA_CMA` stub provided in `dma-map-ops.h`.
- `EXPORT_SYMBOL_GPL` used appropriately.

**Issue - `rmem_cma_areas` is no longer `__initdata`:**

```c
+static struct cma *rmem_cma_areas[MAX_CMA_AREAS];
+static unsigned int rmem_cma_areas_num;
```

The old `dma_areas` in `cma_heap.c` was marked `__initdata`, meaning it was discarded after init. The new `rmem_cma_areas` in `contiguous.c` is *not* marked `__initdata` -- and correctly so, because a module could be loaded after init. However, this means `MAX_CMA_AREAS` worth of pointers now persist for the lifetime of the kernel. This is intentional and correct for module support, but worth noting in the commit message.

**Minor - no locking on `rmem_cma_areas`:**
`rmem_cma_insert_area()` is called from `__init` context (`rmem_cma_setup`), and `dma_contiguous_get_reserved_region()` is called from `module_init`. Since all inserts happen during early boot (before modules load), there's no real race. This is fine.

**Nit:** The function name `dma_contiguous_get_reserved_region` is a good choice and follows the existing naming convention in `contiguous.c`.

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Generated by Claude Code Patch Reviewer