net: ravb: serialize PTP clock teardown

In the Linux kernel, the following vulnerability has been resolved: net: ravb: serialize PTP clock teardown ravb_ptp_interrupt() can race with ravb_ptp_stop() and pass the clock to ptp_clock_event() while ptp_clock_unregister() is freeing it. This can lead to a use-after-free. Use READ_ONCE() and WRITE_ONCE() for lockless access to the clock pointer. Atomically detach it with xchg() before disabling PTP interrupts, then synchronize all IRQs which can invoke ravb_ptp_interrupt() before unregistering the detached clock. A handler which read the old pointer completes before the clock is unregistered, while later handlers read NULL and skip the event.