xen-netback: Add stat counters for zerocopy

These counters help determine how often the buffers had to be copied. Also
they help find out if packets are leaked, as if "sent != success + fail",
there are probably packets never freed up properly.

NOTE: if bisect brought you here, you should apply the series up until
"xen-netback: Timeout packets in RX path", otherwise Windows guests can't work
properly and malicious guests can block other guests by not releasing their sent
packets.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 46a7570..3cb5863 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1323,8 +1323,10 @@
 		 * do a skb_copy_ubufs while we are still in control of the
 		 * skb. E.g. the __pskb_pull_tail earlier can do such thing.
 		 */
-		if (skb_shinfo(skb)->destructor_arg)
+		if (skb_shinfo(skb)->destructor_arg) {
 			skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
+			vif->tx_zerocopy_sent++;
+		}
 
 		netif_receive_skb(skb);
 	}
@@ -1364,6 +1366,11 @@
 		napi_schedule(&vif->napi);
 		local_bh_enable();
 	}
+
+	if (likely(zerocopy_success))
+		vif->tx_zerocopy_success++;
+	else
+		vif->tx_zerocopy_fail++;
 }
 
 static inline void xenvif_tx_dealloc_action(struct xenvif *vif)