virtio_net: free xdp shinfo frags when build_skb_from_xdp_buff() fails

build_skb_from_xdp_buff() may return NULL, in this case
we need to free the frags of xdp shinfo.

Fixes: fab89baf ("virtio-net: support multi-buffer xdp")
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -1273,9 +1273,12 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 
 		switch (act) {
 		case XDP_PASS:
+			head_skb = build_skb_from_xdp_buff(dev, vi, &xdp, xdp_frags_truesz);
+			if (unlikely(!head_skb))
+				goto err_xdp_frags;
+
 			if (unlikely(xdp_page != page))
 				put_page(page);
-			head_skb = build_skb_from_xdp_buff(dev, vi, &xdp, xdp_frags_truesz);
 			rcu_read_unlock();
 			return head_skb;
 		case XDP_TX: