sfc: Add TX merged completion counter

Add a counter for TX merged completion events.

This is implemented in the common TX path, because the NIC event
handlers only know how many descriptors were completed, not how many
packets.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/tx.c b/drivers/net/ethernet/sfc/tx.c
index 4903c4f..85ee647 100644
--- a/drivers/net/ethernet/sfc/tx.c
+++ b/drivers/net/ethernet/sfc/tx.c
@@ -437,6 +437,9 @@
 	efx_dequeue_buffers(tx_queue, index, &pkts_compl, &bytes_compl);
 	netdev_tx_completed_queue(tx_queue->core_txq, pkts_compl, bytes_compl);
 
+	if (pkts_compl > 1)
+		++tx_queue->merge_events;
+
 	/* See if we need to restart the netif queue.  This memory
 	 * barrier ensures that we write read_count (inside
 	 * efx_dequeue_buffers()) before reading the queue status.