Claude review: staging: fbtft: Use sysfs_emit_at() to print to sysfs file

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

Patch Review

**The bug:** In `sprintf_gamma()` at line 101, the original code passes `PAGE_SIZE` as the size limit to `scnprintf()`:

```c
len += scnprintf(&buf[len], PAGE_SIZE,
     "%04x ", curves[i * par->gamma.num_values + j]);
```

Since the write starts at `&buf[len]`, the correct limit should be `PAGE_SIZE - len`. Passing the full `PAGE_SIZE` means `scnprintf` thinks it has more buffer space than actually remains, which could theoretically write past the end of the page-sized sysfs buffer.

**The fix:** Replaces this with `sysfs_emit_at(buf, len, ...)`:

```c
len += sysfs_emit_at(buf, len,
     "%04x ", curves[i * par->gamma.num_values + j]);
```

`sysfs_emit_at()` internally computes `PAGE_SIZE - offset` for the limit and also adds safety checks (page-alignment verification, offset bounds checking), making it the correct and idiomatic API for sysfs show callbacks.

**Correctness:** Verified that this is indeed a sysfs context — `sprintf_gamma()` is called from `show_gamma_curve()` (line 142), which is the show callback registered via `__ATTR(gamma, 0660, show_gamma_curve, store_gamma_curve)` at line 146. The companion function `show_debug()` at line 198 already uses `sysfs_emit()`, so this change also improves consistency within the file.

**Practical impact:** As the commit message correctly notes, the max output is `FBTFT_GAMMA_MAX_VALUES_TOTAL` (128) u32 values formatted as `"%04x "` (5 chars each) = ~640 bytes, well under `PAGE_SIZE` (4096). So this is a correctness fix rather than a security-critical one, but still worth having.

**No issues found.** Reviewed-by worthy.

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Generated by Claude Code Patch Reviewer