s390/futex: Cleanup futex_atomic_cmpxchg_inatomic()

Cleanup the futex_atomic_cmpxchg_inatomic() inline assembly to make it
a bit more readable.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>