Multiple prompts are mandatory for anything non-trivial or/and larger in scope. That said, the exfatprogs repo is ~60k tokens (in 8k LOC) and Linux's exfat driver* is ~40k tokens (in 6k LOC). So directly relevant code is ~100k tokens (in 14k LOC). Not that extensive.

>Not sure if someone can train a model on the kernel code or exfatprogs and generate the code.

They can certainly finetune such a model. Not a crazy idea, just computationally expensive. (But less expensive than training from scratch.)

*Of course Linux driver also uses many includes so if consider those alongside linked code the number goes significantly up.