Bluetooth: Drop L2CAP chan reference if ERTM ack_timer fired

Reference counter was incremented when starting ack timer but
decremented only when clearing timer, not when timer fired.
Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

net/bluetooth/l2cap_core.c

@@ -1996,6 +1996,8 @@ static void l2cap_ack_timeout(struct work_struct *work)
 	lock_sock(chan->sk);
 	__l2cap_send_ack(chan);
 	release_sock(chan->sk);
+
+	l2cap_chan_put(chan);
 }
 
 static inline void l2cap_ertm_init(struct l2cap_chan *chan)