Claude review: lib: add dmabuf token infrastructure

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

Patch Review

The core of the series. The token/map separation is well-motivated for supporting invalidation. The percpu refcount + dma fence pattern for tracking in-flight I/O is correct.

**Bug: Missing cleanup after failed `WARN_ON_ONCE` in `io_dmabuf_token_create`**

```c
ret = file->f_op->create_dmabuf_token(file, token);
if (ret) {
    memset(token, 0, sizeof(*token));
    dma_buf_put(dmabuf);
    return ret;
}

if (WARN_ON_ONCE(!token->dev_ops ||
         !token->dev_ops->map ||
         !token->dev_ops->unmap ||
         !token->dev_ops->release))
    return -EINVAL;
```

If the WARN fires, the function returns `-EINVAL` without calling `dma_buf_put(dmabuf)` or cleaning up whatever `create_dmabuf_token` allocated (via `dev_ops->release`). The dmabuf ref from `get_dma_buf()` earlier is leaked. The token struct may also be partially initialized.

**SPDX identifier:** `lib/io_dmabuf_token.c` uses `/* SPDX-License-Identifier: GPL-2.0 */` (C-style comment) which is for header files. Source files should use `// SPDX-License-Identifier: GPL-2.0`.

**Nit:** The header guard is `_LINUX_DMA_TOKEN_H` but the file is `io_dmabuf_token.h` — the guard could be `_LINUX_IO_DMABUF_TOKEN_H` for consistency.

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