HID: hiddev: Fix race in in hiddev_disconnect()

Syzbot reports that "hiddev" is used after it's free in hiddev_disconnect().
The hiddev_disconnect() function sets "hiddev->exist = 0;" so
hiddev_release() can free it as soon as we drop the "existancelock"
lock.  This patch moves the mutex_unlock(&hiddev->existancelock) until
after we have finished using it.

Reported-by: syzbot+784ccb935f9900cc7c9e@syzkaller.appspotmail.com
Fixes: 7f77897e ("HID: hiddev: fix potential use-after-free")
Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/usbhid/hiddev.c

@@ -932,9 +932,9 @@ void hiddev_disconnect(struct hid_device *hid)
 	hiddev->exist = 0;
 
 	if (hiddev->open) {
-		mutex_unlock(&hiddev->existancelock);
 		hid_hw_close(hiddev->hid);
 		wake_up_interruptible(&hiddev->wait);
+		mutex_unlock(&hiddev->existancelock);
 	} else {
 		mutex_unlock(&hiddev->existancelock);
 		kfree(hiddev);