batman-adv: tp_meter: fix race condition in send error reporting

batman-adv: tp_meter: fix race condition in send error reporting

batadv_tp_sender_shutdown() previously used two separate variables to track
session state: sending (an atomic flag indicating whether the session was
active) and reason (a plain enum storing the stop reason). This introduced
a race window between the two writes: after sending was cleared to 0,
batadv_tp_send() could observe the stopped state and call
batadv_tp_sender_end() before reason was written, causing the wrong stop
reason to be reported to the caller.

Fix this by consolidating both variables into a single atomic send_result,
which holds 0 while the session is running and the stop reason once it
ends.

Cc: stable@kernel.org
Fixes: 33a3bb4a3345 ("batman-adv: throughput meter implementation")
Signed-off-by: Sven Eckelmann <sven@narfation.org>