nptl: clear the whole rseq area before registration

nptl: clear the whole rseq area before registration

Due to the extensible nature of the rseq area we can't explictly
initialize fields that are not part of the ABI yet. It was agreed with
upstream that all new fields will be documented as zero initialized by
userspace. Future kernels configured with CONFIG_DEBUG_RSEQ will
validate the content of all fields during registration.

Replace the explicit field initialization with a memset of the whole
rseq area which will cover fields as they are added to future kernels.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>