[S390] zcrypt: add sanity check before copy_from_user()

It's not obvious that copy_from_user() is called with a sane length
parameter here. Even though it currently seems to be correct better
add a check to prevent stack corruption / exploits.

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