batman-adv: stop tp_meter sessions during mesh teardown

In the Linux kernel, the following vulnerability has been resolved: batman-adv: stop tp_meter sessions during mesh teardown TP meter sessions remain linked on bat_priv->tp_list after the netlink request has already finished. When the mesh interface is removed, batadv_mesh_free() currently tears down the mesh without first draining these sessions. A running sender thread or a late incoming tp_meter packet can then keep processing against a mesh instance which is already shutting down. Synchronize tp_meter with the mesh lifetime by stopping all active sessions from batadv_mesh_free() and waiting for sender threads to exit before teardown continues.