usb: gadget: f_fs: Ensure ep0req is dequeued before free_request

As per the documentation, function usb_ep_free_request guarantees
the request will not be queued or no longer be re-queued (or
otherwise used). However, with the current implementation it
doesn't make sure that the request in ep0 isn't reused.

Fix this by dequeuing the ep0req on functionfs_unbind before
freeing the request to align with the definition.

Fixes: ddf8abd2 ("USB: f_fs: the FunctionFS driver")
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>
Tested-by: Krishna Kurapati <quic_kriskura@quicinc.com>
Link: https://lore.kernel.org/r/20221215052906.8993-3-quic_ugoswami@quicinc.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -1895,6 +1895,8 @@ static void functionfs_unbind(struct ffs_data *ffs)
 	ENTER();
 
 	if (!WARN_ON(!ffs->gadget)) {
+		/* dequeue before freeing ep0req */
+		usb_ep_dequeue(ffs->gadget->ep0, ffs->ep0req);
 		mutex_lock(&ffs->mutex);
 		usb_ep_free_request(ffs->gadget->ep0, ffs->ep0req);
 		ffs->ep0req = NULL;