tcp: restrict the contents of tcp_timer.h to the kernel context

[tuexen]

Summary

The header file contains only stuff relevant in the kernel context.

There is no #warning or #error in case the header file is not included in the kernel context, since then this header file would need to be added to tools/build/test-includes/badfiles.inc, but that list should not grow.

[glebius]

There is no #warning or #error in case the header file is not included in the kernel context, since then this header file would need to be added to tools/build/test-includes/badfiles.inc, but that list should not grow.

Yeah, I also tried that. But that's the whole point, we want it to be a bad include :)

I'll add Warner to get his opinion on how to cover this case. This actually is a common case. We leak a kernel header to userland and after years appears some accidental use of it.

[imp]

This looks good to me. It's the usual way we deal with stuff like this

[glebius]

Thanks, @imp! What if instead of allowing the header but masking its internals, we want to eliminate any use of a header by userland applications? Some headers have #error no user serviceable parts inside and AFAIU all of them are listed in badfiles.inc. Is this the way to go?

[glebius]

We will sort out how to correctly put #error later.