Align struct file_operations with upstream v6.14

[Jordymalone]

Chapter 6, "Character Device Drivers", section 6.1 still showed the v5.4 layout of 'struct file_operations'. Replace it with the version shipped in upstream Linux v6.14, keeping the example buildable and the API in step with modern kernels.

Upstream reference: https://elixir.bootlin.com/linux/v6.14/source/include/linux/fs.h#L2131

Summary by Bito

This pull request updates the 'struct file_operations' documentation to align with Linux kernel version 6.14, replacing the outdated reference to version 5.4. Enhancements include modifications to function pointers and the addition of new fields, improving API clarity for character device driver developers.

Unit tests added: False

Estimated effort to review (1-5, lower is better): 2