tipc: fix UAF in tipc_l2_send_msg()

In the Linux kernel, the following vulnerability has been resolved: tipc: fix UAF in tipc_l2_send_msg() Syzbot reported a slab-use-after-free in ipvlan_hard_header() when called from tipc_l2_send_msg(). The root cause is that tipc_disable_l2_media() calls synchronize_net() while b->media_ptr is still valid. This allows concurrent RCU readers to obtain the device pointer after synchronize_net() has finished. The pointer is cleared later in bearer_disable(), but without any subsequent synchronization, allowing the device to be freed while still in use by readers. Fix this by clearing b->media_ptr in tipc_disable_l2_media() before calling synchronize_net(). This is safe to do now because the call order in bearer_disable() was reversed in 0d051bf93c06 ("tipc: make bearer packet filtering generic") to call tipc_node_delete_links() (which needs the pointer) before disable_media(). https: //lore.kernel.org/netdev/6a2c1007.428ffe26.258b27.015d.GAE@google.com/T/#u