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

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

Patch Review

Straightforward change from `bool` to `tristate` and adding `MODULE_LICENSE`/`MODULE_IMPORT_NS`.

**Issue: Missing `MODULE_LICENSE` should have been in patch 5**

Patch 5 adds `MODULE_DESCRIPTION` but not `MODULE_LICENSE("GPL")`. Even for built-in code, it's good practice. More importantly, this means patch 5 doesn't compile as a module (the Kconfig is `bool` in patch 5, so it's technically fine, but it's cleaner to include the license from the start).

**No `module_exit`**: As noted above, this will warn when built as a module. The CMA heap series presumably addresses this the same way, but it should be explicitly noted or suppressed.

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### Summary of key issues:

1. **Bug (patch 2)**: Double `put_device()` in `dma_heap_register()` error path + `dma_heap_add()` error path causes UAF
2. **Bug (patch 2)**: `dma_heap_dev_release()` does `kfree(heap->name)` but `dma_heap_create()` doesn't `kstrdup()` the name — existing callers pass string literals
3. **Bug (patch 2 + 5)**: `err3` path in `dma_heap_register()` does `device_destroy()` + `dma_heap_destroy()` — double put/free
4. **Bug (patch 5)**: Double-free of `coh_heap->name` in `__coherent_heap_register()` error path
5. **Concern (patch 5)**: `virt_to_page()` on `dma_alloc_coherent()` return is unsafe on architectures with `no-map` coherent regions
6. **Minor (patch 5)**: Unnecessary cache sync ops for coherent memory, swallowed error code in `map_dma_buf`, missing `\n` in `pr_warn`

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