Claude review: dma-buf: heaps: use max3() in dma_heap_ioctl

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

Patch Review

**Status: Looks good**

The patch does two things:

1. Adds `#include <linux/minmax.h>` — correct, since `max3()` is defined there and was not previously included directly. Alphabetical ordering of the include is maintained.

2. Replaces:
   ```c
   ksize = max(max(in_size, out_size), drv_size);
   ```
   with:
   ```c
   ksize = max3(in_size, out_size, drv_size);
   ```

This is a straightforward substitution. `max3()` in `include/linux/minmax.h` (line 152) is defined via `__careful_op3(max, ...)` which performs the same type-safe three-way max. The semantics are identical.

**Minor observations:**
- The v1→v2 changelog is clear: v1 was missing the explicit `minmax.h` include, and T.J. Mercier caught it. Good iteration.
- `max()` is likely already transitively available (via `kernel.h` or similar), but adding the explicit `minmax.h` include for `max3()` is the right practice — it avoids relying on transitive includes that could break in future header cleanups.

No issues found. The patch is correct and ready to merge.

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