s390/cmpxchg: add missing memory barrier to cmpxchg64

All cmpxchg functions imply a memory barrier.
cmpxch64 did not have one for 31 bit code, so add it.

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