vfio: return -ENOTTY for unsupported device feature

The two implementers of vfio_device_ops.device_feature,
vfio_cdx_ioctl_feature and vfio_pci_core_ioctl_feature, return
-ENOTTY in the fallthrough case when the feature is unsupported. For
consistency, the base case, vfio_ioctl_device_feature, should do the
same when device_feature == NULL, indicating an implementation has no
feature extensions.

Signed-off-by: Alex Mastro <amastro@fb.com>
Link: https://lore.kernel.org/r/20250908-vfio-enotty-v1-1-4428e1539e2e@fb.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: Alex Mastro

drivers/vfio/vfio_main.c

@@ -1252,7 +1252,7 @@ static int vfio_ioctl_device_feature(struct vfio_device *device,
 			feature.argsz - minsz);
 	default:
 		if (unlikely(!device->ops->device_feature))
-			return -EINVAL;
+			return -ENOTTY;
 		return device->ops->device_feature(device, feature.flags,
 						   arg->data,
 						   feature.argsz - minsz);