xen-netback: fix extra_info handling in xenvif_tx_err()

Patch 562abd39 "xen-netback: support multiple extra info fragments
passed from frontend" contained a mistake which can result in an in-
correct number of responses being generated when handling errors
encountered when processing packets containing extra info fragments.
This patch fixes the problem.

Signed-off-by: Paul Durrant <[email protected]>
Reported-by: Jan Beulich <[email protected]>
Cc: Wei Liu <[email protected]>
Acked-by: Wei Liu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index b42f260..4412a57 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -711,6 +711,7 @@
 		if (cons == end)
 			break;
 		RING_COPY_REQUEST(&queue->tx, cons++, txp);
+		extra_count = 0; /* only the first frag can have extras */
 	} while (1);
 	queue->tx.req_cons = cons;
 }