tuntap: simplify error handling in tun_build_skb()

There's no need to duplicate page get logic in each action. So this
patch tries to get page and calculate the offset before processing XDP
actions (except for XDP_DROP), and undo them when meet errors (we
don't care the performance on errors). This will be used for factoring
out XDP logic.
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1701,17 +1701,13 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 			xdp_do_flush_map();
 			if (err)
 				goto err_redirect;
-			rcu_read_unlock();
-			local_bh_enable();
-			return NULL;
+			goto out;
 		case XDP_TX:
 			get_page(alloc_frag->page);
 			alloc_frag->offset += buflen;
 			if (tun_xdp_tx(tun->dev, &xdp) < 0)
 				goto err_redirect;
-			rcu_read_unlock();
-			local_bh_enable();
-			return NULL;
+			goto out;
 		case XDP_PASS:
 			delta = orig_data - xdp.data;
 			len = xdp.data_end - xdp.data;
@@ -1742,7 +1738,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 
 err_redirect:
 	put_page(alloc_frag->page);
-err_xdp:
+out:
 	rcu_read_unlock();
 	local_bh_enable();
 	this_cpu_inc(tun->pcpu_stats->rx_dropped);