s390/vdso: Use generic union vdso_data_store

There is already a generic union definition for vdso_data_store in the vdso
datapage header.

Use this definition to prevent code duplication.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20240219153939.75719-8-anna-maria@linutronix.de