virtio_net: small: avoid code duplication in xdp scenarios

Avoid the problem that some variables(headroom and so on) will repeat
the calculation when process xdp.
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/virtio_net.c

@@ -1031,11 +1031,10 @@ static struct sk_buff *receive_small(struct net_device *dev,
 	struct sk_buff *skb;
 	struct bpf_prog *xdp_prog;
 	unsigned int xdp_headroom = (unsigned long)ctx;
-	unsigned int header_offset = VIRTNET_RX_PAD + xdp_headroom;
-	unsigned int headroom = vi->hdr_len + header_offset;
-	unsigned int buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
-			      SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 	struct page *page = virt_to_head_page(buf);
+	unsigned int header_offset;
+	unsigned int headroom;
+	unsigned int buflen;
 
 	len -= vi->hdr_len;
 	stats->bytes += len;
@@ -1063,6 +1062,11 @@ static struct sk_buff *receive_small(struct net_device *dev,
 	rcu_read_unlock();
 
 skip_xdp:
+	header_offset = VIRTNET_RX_PAD + xdp_headroom;
+	headroom = vi->hdr_len + header_offset;
+	buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
+		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
 	skb = build_skb(buf, buflen);
 	if (!skb)
 		goto err;