s390/bitops: make test functions return bool

Make s390/bitops test functions return bool values. That enforces return
value range to 0 and 1 and matches with asm-generic/bitops-instrumented.h
declarations as well as some other architectures implementations.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>