usb: gadget: uvc: configfs: Allocate groups dynamically

The UVC configfs implementation creates all groups as global static
variables. This prevents creation of multiple UVC function instances,
as they would all require their own configfs group instances.

Fix this by allocating all groups dynamically. To avoid duplicating code
around, extend the config_item_type structure with group name and
children, and implement helper functions to create children
automatically for most groups.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

drivers/usb/gadget/function/f_uvc.c

@@ -792,6 +792,7 @@ static struct usb_function_instance *uvc_alloc_inst(void)
 	struct uvc_output_terminal_descriptor *od;
 	struct uvc_color_matching_descriptor *md;
 	struct uvc_descriptor_header **ctl_cls;
+	int ret;
 
 	opts = kzalloc(sizeof(*opts), GFP_KERNEL);
 	if (!opts)
@@ -868,7 +869,12 @@ static struct usb_function_instance *uvc_alloc_inst(void)
 	opts->streaming_interval = 1;
 	opts->streaming_maxpacket = 1024;
 
-	uvcg_attach_configfs(opts);
+	ret = uvcg_attach_configfs(opts);
+	if (ret < 0) {
+		kfree(opts);
+		return ERR_PTR(ret);
+	}
+
 	return &opts->func_inst;
 }