rseq: Update kernel fields in lockstep with CONFIG_DEBUG_RSEQ=y

With CONFIG_DEBUG_RSEQ=y, an in-kernel copy of the read-only fields is
kept synchronized with the user-space fields. Ensure the updates are
done in lockstep in case we error out on a write to user-space.

Fixes: 7d5265ff ("rseq: Validate read-only fields under DEBUG_RSEQ config")
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20250225202500.731245-1-mjeanson@efficios.com