s390/zcrypt: use strscpy() to instead of strncpy()

The implementation of strscpy() is more robust and safer.
That's now the recommended way to copy NUL-terminated strings.

Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
Link: https://lore.kernel.org/r/202301052024349365834@zte.com.cn


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>