[COMMITTED 2.40 1/2] nptl: initialize rseq area prior to registration

[Florian Weimer]

* Michael Jeanson via Libc-stable:

> Per the rseq syscall documentation, 3 fields are required to be
> initialized by userspace prior to registration, they are 'cpu_id',
> 'rseq_cs' and 'flags'. Since we have no guarantee that 'struct pthread'
> is cleared on all architectures, explicitly set those 3 fields prior to
> registration.
>
> Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
> Reviewed-by: Florian Weimer <fweimer@redhat.com>
> (cherry picked from commit 97f60abd25628425971f07e9b0e7f8eec0741235)

Should we backport this further, maybe even to 2.35?

Thanks,
Florian