[vfs] overlayfs: decref VD when not using it.

overlay/filesystem.go:lookupLocked() did not DecRef the VD on some error paths
when it would not end up saving or using the VD.

PiperOrigin-RevId: 330589742

pkg/sentry/fsimpl/overlay/filesystem.go

@@ -211,6 +211,7 @@ func (fs *filesystem) lookupLocked(ctx context.Context, parent *dentry, name str
 			lookupErr = err
 			return false
 		}
+		defer childVD.DecRef(ctx)
 
 		mask := uint32(linux.STATX_TYPE)
 		if !existsOnAnyLayer {
@@ -249,6 +250,7 @@ func (fs *filesystem) lookupLocked(ctx context.Context, parent *dentry, name str
 		}
 
 		// Update child to include this layer.
+		childVD.IncRef()
 		if isUpper {
 			child.upperVD = childVD
 			child.copiedUp = 1