[RFC PATCH glibc] Linux: Use fixed rseq_len value for rseq registration

[Florian Weimer]

* Christian Brauner:

>> It works reliably as long as glibc only ever uses the minimum rseq size.
>> And since glibc monopolizes the rseq registration, applications cannot
>> register a larger area.  So there is no way to make use of any future
>> kernel extensions.
>
> But when you bump ABI in glibc you can switch to a new rseq size, no?

We are expected to support interposers with their own definition
__rseq_abi, which could be smaller.

Thanks,
Florian