xen-netback: support multiple extra info fragments passed from frontend

The code does not currently support a frontend passing multiple extra info
fragments to the backend in a tx request. The xenvif_get_extras() function
handles multiple extra_info fragments but make_tx_response() assumes there
is only ever a single extra info fragment.

This patch modifies xenvif_get_extras() to pass back a count of extra
info fragments, which is then passed to make_tx_response() (after
possibly being stashed in pending_tx_info for deferred responses).

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index 1128252..f44b388 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h
@@ -52,6 +52,7 @@
 
 struct pending_tx_info {
 	struct xen_netif_tx_request req; /* tx request */
+	unsigned int extra_count;
 	/* Callback data for released SKBs. The callback is always
 	 * xenvif_zerocopy_callback, desc contains the pending_idx, which is
 	 * also an index in pending_tx_info array. It is initialized in