s390/uvdevice: Add Retrieve Secret IOCTL

s390/uvdevice: Add Retrieve Secret IOCTL

Add a new IOCL number to support the new Retrieve Secret UVC for
user-space.
User-space provides the index of the secret (u16) to retrieve.
The uvdevice calls the Retrieve Secret UVC and copies the secret into
the provided buffer if it fits. To get the secret type, index, and size
user-space needs to call the List UVC first.

Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Link: https://lore.kernel.org/r/20241024062638.1465970-4-seiden@linux.ibm.com
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>