tun: remove unnecessary check in tun_flow_update

caller has guaranted that rxhash is not zero
Signed-off-by: NLi RongQing <lirongqing@baidu.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -529,10 +529,7 @@ static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
 	unsigned long delay = tun->ageing_time;
 	u16 queue_index = tfile->queue_index;
 
-	if (!rxhash)
-		return;
-	else
-		head = &tun->flows[tun_hashfn(rxhash)];
+	head = &tun->flows[tun_hashfn(rxhash)];
 
 	rcu_read_lock();