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

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

Patch Review

This patch inverts the CMA heap registration: instead of `rmem_cma_setup()` calling into `cma_heap.c` to register each reserved region, it stores them in `rmem_cma_areas[]` inside `contiguous.c` and provides an iterator `dma_contiguous_get_reserved_region()` for the heap to use.

**Bug (build break):** The patch removes the `dma_heap_cma_register_heap()` function definition from `cma_heap.c`, but fails to remove:

1. The call at `kernel/dma/contiguous.c:276`:
```c
ret = dma_heap_cma_register_heap(dma_contiguous_default_area);
if (ret)
    pr_warn("Couldn't register default CMA heap.");
```

2. The header `include/linux/dma-buf/heaps/cma.h` which still declares the function:
```c
#ifdef CONFIG_DMABUF_HEAPS_CMA
int dma_heap_cma_register_heap(struct cma *cma);
#else
static inline int dma_heap_cma_register_heap(struct cma *cma)
{
    return 0;
}
#endif
```

3. The `#include <linux/dma-buf/heaps/cma.h>` at `kernel/dma/contiguous.c:45`.

When `CONFIG_DMABUF_HEAPS_CMA=y`, the `#ifdef` in the header selects the non-inline declaration, but there is no corresponding definition in any `.c` file, resulting in a linker error. This call is now functionally redundant anyway, since `add_cma_heaps()` already retrieves the default CMA area via `dev_get_cma_area(NULL)`.

**Fix:** Remove the call at `contiguous.c:276-278`, remove the `include/linux/dma-buf/heaps/cma.h` header entirely (it has no remaining purpose), and remove the `#include` at `contiguous.c:45`.

The series is **not bisectable** at this patch as-is (`CONFIG_DMABUF_HEAPS_CMA=y` breaks from this point onward).

The new `rmem_cma_areas[]` and `dma_contiguous_get_reserved_region()` implementation itself looks correct. The array is correctly not marked `__initdata` since it must persist for the module to query at load time.

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