driver core: fix potential NULL pointer dereference in dev_uevent()

In the Linux kernel, the following vulnerability has been resolved: driver core: fix potential NULL pointer dereference in dev_uevent() If userspace reads "uevent" device attribute at the same time as another threads unbinds the device from its driver, change to dev->driver from a valid pointer to NULL may result in crash. Fix this by using READ_ONCE() when fetching the pointer, and take bus' drivers klist lock to make sure driver instance will not disappear while we access it. Use WRITE_ONCE() when setting the driver pointer to ensure there is no tearing.