HID: hiddev: fix memory corruption due to invalid intfdata

Commit bd25f4dd ("HID: hiddev: use usb_find_interface,
get rid of BKL") introduced using of private intfdata in hiddev for
purpose of storing hiddev pointer.

This is a problem, because intf pointer is already being set to struct
hid_device pointer by HID core. This obviously lead to memory corruptions
at device disconnect time, such as

WARNING: at lib/kobject.c:595 kobject_put+0x37/0x4b()
kobject: '(null)' (ffff88011e9cd898): is not initialized, yet kobject_put() is being called.

Convert hiddev into accessing hiddev through struct hid_device which is
in intfdata already.
Reported-and-tested-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Reported-and-tested-by: Heinz Diehl <htd@fritha.org>
Reported-and-tested-by: Alan Ott <alan@signal11.us>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/usbhid/hiddev.c

@@ -266,13 +266,15 @@ static int hiddev_open(struct inode *inode, struct file *file)
 {
 	struct hiddev_list *list;
 	struct usb_interface *intf;
+	struct hid_device *hid;
 	struct hiddev *hiddev;
 	int res;
 
 	intf = usb_find_interface(&hiddev_driver, iminor(inode));
 	if (!intf)
 		return -ENODEV;
-	hiddev = usb_get_intfdata(intf);
+	hid = usb_get_intfdata(intf);
+	hiddev = hid->hiddev;
 
 	if (!(list = kzalloc(sizeof(struct hiddev_list), GFP_KERNEL)))
 		return -ENOMEM;
@@ -890,7 +892,6 @@ int hiddev_connect(struct hid_device *hid, unsigned int force)
 	hid->hiddev = hiddev;
 	hiddev->hid = hid;
 	hiddev->exist = 1;
-	usb_set_intfdata(usbhid->intf, usbhid);
 	retval = usb_register_dev(usbhid->intf, &hiddev_class);
 	if (retval) {
 		err_hid("Not able to get a minor for this device.");