s390/zcrypt: add copy_from_user length plausibility checks

There have been identified some places in the zcrypt
device driver where copy_from_user() is called but the
length value is not explicitly checked.

So now some plausibility checks and comments have been
introduced there.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>