vfio: Avoid repeated user pointer cast in vfio_device_fops_unl_ioctl()

This adds a local variable to store the user pointer cast result from arg.
It avoids the repeated casts in the code when more ioctls are added.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Yanting Jiang <yanting.jiang@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230718135551.6592-22-yi.l.liu@intel.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_main.c

@@ -1146,6 +1146,7 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
 {
 	struct vfio_device_file *df = filep->private_data;
 	struct vfio_device *device = df->device;
+	void __user *uptr = (void __user *)arg;
 	int ret;
 
 	/* Paired with smp_store_release() following vfio_df_open() */
@@ -1158,7 +1159,7 @@ static long vfio_device_fops_unl_ioctl(struct file *filep,
 
 	switch (cmd) {
 	case VFIO_DEVICE_FEATURE:
-		ret = vfio_ioctl_device_feature(device, (void __user *)arg);
+		ret = vfio_ioctl_device_feature(device, uptr);
 		break;
 
 	default: