batman-adv: fix tp_meter counter underflow during shutdown

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

batman-adv: fix tp_meter counter underflow during shutdown

batadv_tp_sender_shutdown() unconditionally decrements the "sending"

atomic counter. If multiple paths (e.g. timeout, user cancel, and

normal finish) call this function, the counter can underflow to -1.

Since the sender logic treats any non-zero value as "still sending",

a negative value causes the sender kthread to loop indefinitely.

This leads to a use-after-free when the interface is removed while

the zombie thread is still active.

Fix this by using atomic_xchg() to ensure the counter only transitions

from 1 to 0 once.

[sven: added missing change in batadv_tp_send]