tuntap: enable bh early during processing XDP

This patch move the bh enabling a little bit earlier, this will be
used for factoring out the core XDP logic of tuntap.
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1726,22 +1726,18 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
 			goto err_xdp;
 		}
 	}
+	rcu_read_unlock();
+	local_bh_enable();
 
 	skb = build_skb(buf, buflen);
-	if (!skb) {
-		rcu_read_unlock();
-		local_bh_enable();
+	if (!skb)
 		return ERR_PTR(-ENOMEM);
-	}
 
 	skb_reserve(skb, pad - delta);
 	skb_put(skb, len);
 	get_page(alloc_frag->page);
 	alloc_frag->offset += buflen;
 
-	rcu_read_unlock();
-	local_bh_enable();
-
 	return skb;
 
 err_redirect: