tun: fix rcu_read_lock imbalance in tun_build_skb

rcu_read_lock in tun_build_skb is used to rcu_dereference tun->xdp_prog
safely, rcu_read_unlock should be done in every return path.

Now I could see one place missing it, where it returns NULL in switch-case
XDP_REDIRECT,  another palce using rcu_read_lock wrongly, where it returns
NULL in if (xdp_xmit) chunk.

So fix both in this patch.

Fixes: 761876c8 ("tap: XDP support")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1485,6 +1485,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 			err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
 			if (err)
 				goto err_redirect;
+			rcu_read_unlock();
 			return NULL;
 		case XDP_TX:
 			xdp_xmit = true;
@@ -1517,7 +1518,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 	if (xdp_xmit) {
 		skb->dev = tun->dev;
 		generic_xdp_tx(skb, xdp_prog);
-		rcu_read_lock();
+		rcu_read_unlock();
 		return NULL;
 	}