Claude review: dma-buf: dma-heap: Keep track of the heap device struct

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

Patch Review

This patch stores `heap->heap_dev` and adds a `dma_heap_get_dev()` accessor. Originally from John Stultz with Maxime's Reviewed-by.

**Issue: Double blank line**

```c
+	/* Make sure it doesn't disappear on us */
+	heap->heap_dev = get_device(heap->heap_dev);
+
+
 	/* Add heap to the list */
```

There's an extra blank line (cosmetic, minor). This gets removed in patch 2 anyway, but still worth noting for the patch itself.

**Observation**: The `get_device()` call is done *after* the duplicate-name check loop, which means if the name check passes, we do `get_device()`. But if we fail anywhere after `device_create()` in err3, we do `device_destroy()` without a corresponding `put_device()`. This asymmetry is benign in the current patch because `device_destroy()` does the cleanup, but the patch 2 refactoring changes this significantly. Fine for bisectability since patch 2 immediately reworks this.

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