net/packet: fix a race in packet_set_ring() and packet_notifier()

In the Linux kernel, the following vulnerability has been resolved:

net/packet: fix a race in packet_set_ring() and packet_notifier()

When packet_set_ring() releases po->bind_lock, another thread can run packet_notifier() and process an NETDEV_UP event.

This race and the fix are both similar to that of commit 15fe076edea7 ("net/packet: fix a race in packet_bind() and packet_notifier()").

There too the packet_notifier NETDEV_UP event managed to run while a po->bind_lock critical section had to be temporarily released. And the fix was similarly to temporarily set po->num to zero to keep the socket unhooked until the lock is retaken.

The po->bind_lock in packet_set_ring and packet_notifier precede the introduction of git history.