s390/zcrypt: Use kmemdup to replace kmalloc + memcpy

kmemdup has implemented the function that kmalloc() + memcpy() will
do. We prefer to use the kmemdup function rather than an open coded
implementation.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>