sfc: Insert read memory barrier after checking MAC statistics flag

This prevents speculative reading of the statistics before the
completion flag.

From: Neil Turton <nturton@solarflare.com>
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/sfc/falcon.c

diff --git a/drivers/net/sfc/falcon.c b/drivers/net/sfc/falcon.c
index 70c681e..3a5289f 100644
--- a/drivers/net/sfc/falcon.c
+++ b/drivers/net/sfc/falcon.c
@@ -1953,8 +1953,10 @@
 
 	/* Wait for transfer to complete */
 	for (i = 0; i < 400; i++) {
-		if (*(volatile u32 *)dma_done == FALCON_STATS_DONE)
+		if (*(volatile u32 *)dma_done == FALCON_STATS_DONE) {
+			rmb(); /* Ensure the stats are valid. */
 			return 0;
+		}
 		udelay(10);
 	}