Claude review: drm/virtgpu: Use ERR_PTR for fb_create callback error return

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

Patch Review

**Correctness: Good.** The `fb_create` callback's return contract is documented in `include/drm/drm_mode_config.h:78-81`:

> RETURNS: A new framebuffer with an initial reference count of 1 or a negative error code encoded with ERR_PTR().

The caller in the DRM core will use `IS_ERR()` to check the return value. Returning `NULL` instead of `ERR_PTR()` means the caller would not detect the error — it would treat `NULL` as a valid framebuffer pointer, leading to a null pointer dereference.

**Consistency: Good.** Looking at the patched function (`virtgpu_display.c:318-351`), the three other error paths in `virtio_gpu_user_framebuffer_create` already return `ERR_PTR()` correctly:

```c
return ERR_PTR(-ENOENT);   // line 330 - bad pixel format
return ERR_PTR(-EINVAL);   // line 335 - object lookup failure
return ERR_PTR(-ENOMEM);   // line 340 - allocation failure
```

Only the `virtio_gpu_framebuffer_init` failure path (the one being fixed) was inconsistent, returning `NULL`. The fix changes it to `ERR_PTR(ret)`, which correctly propagates the actual error code from `virtio_gpu_framebuffer_init`.

**No issues found.** Clean, minimal, correct fix.

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