Claude review: fbdev: defio: Move variable state into struct fb_deferred_io_state

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

Patch Review

Clean refactoring moving `open_count`, `mapping`, and `pagereflist` from the driver-visible `struct fb_deferred_io` to the internal `struct fb_deferred_io_state`.

**Good improvements:**

- `BUG_ON(!info->fbdefio->mapping)` replaced with `fb_WARN_ON_ONCE(info, !fbdefio_state->mapping)` — converting BUG_ON to WARN is the right call here.
- `WARN_ON(!list_empty(&fbdefio_state->pagereflist))` added in `fb_deferred_io_state_release` — good sanity check.
- `INIT_LIST_HEAD(&fbdefio_state->pagereflist)` properly moved to `fb_deferred_io_state_alloc`.

**Minor nit:** The added forward declaration:

```c
struct address_space;
```

is technically redundant since `<linux/pagemap.h>` is already included (which pulls in `<linux/fs.h>` where `struct address_space` is defined). It's harmless and documents the dependency, but could be dropped.

The removal of `fbdefio->mapping = NULL` from `fb_deferred_io_cleanup` is correct since `mapping` is now in the state struct, and the state's `info` pointer is set to NULL under the lock, which prevents any access to `mapping` after cleanup.

No issues with this patch.

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