virtio-net: fix leaking page for gso packet during mergeable XDP

We need to drop refcnt to xdp_page if we see a gso packet. Otherwise
it will be leaked. Fixing this by moving the check of gso packet above
the linearizing logic. While at it, remove useless comment as well.

Cc: John Fastabend <john.fastabend@gmail.com>
Fixes: 72979a6c3590 ("virtio_net: xdp, add slowpath case for non contiguous buffers")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -707,6 +707,13 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 		void *data;
 		u32 act;
 
+		/* Transient failure which in theory could occur if
+		 * in-flight packets from before XDP was enabled reach
+		 * the receive path after XDP is loaded.
+		 */
+		if (unlikely(hdr->hdr.gso_type))
+			goto err_xdp;
+
 		/* This happens when rx buffer size is underestimated
 		 * or headroom is not enough because of the buffer
 		 * was refilled before XDP is set. This should only
@@ -727,14 +734,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			xdp_page = page;
 		}
 
-		/* Transient failure which in theory could occur if
-		 * in-flight packets from before XDP was enabled reach
-		 * the receive path after XDP is loaded. In practice I
-		 * was not able to create this condition.
-		 */
-		if (unlikely(hdr->hdr.gso_type))
-			goto err_xdp;
-
 		/* Allow consuming headroom but reserve enough space to push
 		 * the descriptor on if we get an XDP_TX return code.
 		 */