igb: defeature tx head writeback

This patch removes tx head writeback as it was found to not provide a
significant improvement in tx performance and on some systems has been seen
to cause a performance degredation due to partial cache line writes.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/igb/igb.h b/drivers/net/igb/igb.h
index 2121b8b..c906325 100644
--- a/drivers/net/igb/igb.h
+++ b/drivers/net/igb/igb.h
@@ -43,8 +43,6 @@
 #endif
 
 /* Interrupt defines */
-#define IGB_MAX_TX_CLEAN 72
-
 #define IGB_MIN_DYN_ITR 3000
 #define IGB_MAX_DYN_ITR 96000
 
@@ -127,7 +125,8 @@
 		/* TX */
 		struct {
 			unsigned long time_stamp;
-			u32 length;
+			u16 length;
+			u16 next_to_watch;
 		};
 		/* RX */
 		struct {