s390/cache: remove superfluous locking

With "s390/cpuinfo: simplify locking and skip offline cpus early" we
prevent already that cpus will go away. The additional
get_online_cpus() / put_online_cpus() within show_cacheinfo() is not
needed anymore.

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