s390/zcrypt: rework arrays with length zero occurrences

Review and rework all the zero length array occurrences
within structs to flexible array fields or comment if
not used at all. However, some struct fields are there
for documentation purpose or to have correct sizeof()
evaluation of a struct and thus should not get deleted.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>