CVE-2026-64093: batman-adv: tp_meter: directly shut down timer on cleanup

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

batman-adv: tpmeter: directly shut down timer on cleanup

batadvtpsendercleanup() was calling timerdeletesync() followed by timerdelete() to guard against the timer handler re-arming itself between the two calls. This double-deletion hack relied on the sending status being set to 0 to suppress re-arming.

Replace both calls with a single timershutdownsync(). This function both waits for any running timer callback to complete (like timerdeletesync()) and permanently disarms the timer so it cannot be re-armed afterwards, making re-arming prevention unconditional and self-documenting.

The re-arming property is also required because otherwise:

1. context 0 (batadvtprecvack()) checks in batadvtpresetsendertimer() if sending is still 1 -> it is 2. context 1 changes in batadvtpsendershutdown() sending to 0 and in this process forces the kthread to stop timer in batadvtpsendercleanup() 3. context 0 continues in batadvtpresetsendertimer() and rearms the timer -> but the reference for it is already gone