s390/cpum_sf: Convert to cmpxchg128()

Now that there is a cross arch u128 and cmpxchg128(), use those
instead of the custom CDSG helper.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20230531132324.058821078@infradead.org