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