batman-adv: tp_meter: initialize dec_cwnd explicitly

batman-adv: tp_meter: initialize dec_cwnd explicitly

When batadv_tp_update_cwnd() is called, dec_cwnd is increased. But dec_cwnd
is only initialixed (to 0) when a duplicate Ack was received or when cwnd
is below the ss_threshold.

Just initialize the cwnd during the initialization to avoid any potential
access of uninitialized data.

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