s390/cacheinfo: add missing facility check

Git commit d97d929f ("s390: move cacheinfo sysfs to generic cacheinfo
infrastructure") removed the general-instructions-extension availability
check before the ecag instruction is executed.
Without this check this may lead to crashes on machines without this facility.
Therefore add the check again where needed.

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