USB: core: limit access to rawdescriptors which were not allocated

When using systemcall to read the rawdescriptors, make sure we won't
access to the rawdescriptors never allocated, which are number
exceed the USB_MAXCONFIG.

Reported-by: syzbot+256e56ddde8b8957eabd@syzkaller.appspotmail.com
Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>

drivers/usb/core/sysfs.c

@@ -895,7 +895,8 @@ read_descriptors(struct file *filp, struct kobject *kobj,
 	 * configurations (config plus subsidiary descriptors).
 	 */
 	for (cfgno = -1; cfgno < udev->descriptor.bNumConfigurations &&
-			nleft > 0; ++cfgno) {
+			nleft > 0 &&
+			cfgno < USB_MAXCONFIG; ++cfgno) {
 		if (cfgno < 0) {
 			src = &udev->descriptor;
 			srclen = sizeof(struct usb_device_descriptor);