s390/cmpxchg: simplify cmpxchg_double

Since sizeof(long) == 4 is always false now, simplify cmpxchg_double a bit.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>