tg3: restore rx_dropped accounting

commit 511d22247be7 (tg3: 64 bit stats on all arches), overlooked the
rx_dropped accounting.

We use a full "struct rtnl_link_stats64" to hold rx_dropped value, but
forgot to report it in tg3_get_stats64().

Use an "unsigned long" instead to shrink "struct tg3" by 176 bytes, and
report this value to stats readers.

Increment rx_dropped counter for oversized frames.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Michael Chan <mchan@broadcom.com>
CC: Matt Carlson <mcarlson@broadcom.com>
Acked-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tg3.h b/drivers/net/tg3.h
index 4937bd1..be7ff13 100644
--- a/drivers/net/tg3.h
+++ b/drivers/net/tg3.h
@@ -2759,7 +2759,7 @@
 
 
 	/* begin "everything else" cacheline(s) section */
-	struct rtnl_link_stats64	net_stats;
+	unsigned long			rx_dropped;
 	struct rtnl_link_stats64	net_stats_prev;
 	struct tg3_ethtool_stats	estats;
 	struct tg3_ethtool_stats	estats_prev;