sys/linux: add IPPROTO_MPTCP support

[cpaasch]

Note: I can imagine that I should run "make extract,..." as part of this PR here. The problem is that I have some troubles getting the cross-compilers with retpoline on my "syzkaller-system". I can only generate things for amd64 right now. Let me know if that's a big deal, or if you can simply run the make extract,...

Thanks!

Adding support for IPPROTO_MPTCP. Like SMC, it is on top of TCP and can
thus be treated like a regular TCP socket.

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Before sending a pull request, please review Contribution Guidelines:
https://github.com/google/syzkaller/blob/master/docs/contributing.md

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[dvyukov]

Please also check-in the auto-generated files:
https://github.com/google/syzkaller/blob/master/docs/syscall_descriptions.md#describing-new-system-calls
CI is not happy.

Is there a literal constant for that 262? I guess it's not not people are supposed to write in their C code. If there is, please use it and re-run make extract on latest linux-next to update const files.

[dvyukov]

Oh, now I see your Note.
make install_prerequiresites is supposed to install the cross-compilers, but that's obviously won't work on just any linux system.
What do you mean by "syzkaller-system"? You don't need to run make extract on a test system with latest kernel. I always run it on my desktop running stock distro with SOURCEDIR pointing to latest linux-next checkout.
In the end you may edit .const files by hand. Though, it's not recommended and later somebody will overwrite everything with result of auto-generation. But they were specifically designed to be simple text files that can be written manually and edited.

[cpaasch]

“syzkaller-system” is the Linux-server I have at hand which is not apt-based and does not have easy access to the right software packages (aka, cross-compilers)

Anyways, I think I can get my hands on an apt-based host without any package-restrictions.

[cpaasch]

CI-checks failed with

fatal error: runtime: out of memory

[dvyukov]

CI-checks failed with
fatal error: runtime: out of memory

This is not directly related to your change. I am working on fixing this:
#1580
but it's not trivial to fix, pretty inherent problem (size of syscall descriptions grows, memory consumption grows, CI has fixed small amount of memory).

[dvyukov]

I will merge this now and then see if I can easily re-generate consts on linux-next.
Thanks

[dvyukov]

Here we go: 692266c