NIU: Implement discard counters, optimize

Optimize the lightly loaded case, by only synchronizing discards stats
when qlen > 10 indicate potential for drops.

Notice Robert Olsson might disagree with this patch.
Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/niu.c

@@ -3618,7 +3618,9 @@ static int niu_rx_work(struct niu *np, struct rx_ring_info *rp, int budget)
 
 	nw64(RX_DMA_CTL_STAT(rp->rx_channel), stat);
 
-	niu_sync_rx_discard_stats(np, rp, 0x7FFF);
+	/* Only sync discards stats when qlen indicate potential for drops */
+	if (qlen > 10)
+		niu_sync_rx_discard_stats(np, rp, 0x7FFF);
 
 	return work_done;
 }