s390/uvdevice: Add info IOCTL

Add an IOCTL that allows userspace to find out which IOCTLs the uvdevice
supports without trial and error.

Explicitly expose the IOCTL nr for the request types.

Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Link: https://lore.kernel.org/r/20230615100533.3996107-3-seiden@linux.ibm.com


Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20230615100533.3996107-3-seiden@linux.ibm.com>