net: add a comment on netdev->last_rx

As some driver authors seem to reintroduce dev->last_rx use,
add a comment to strongly discourage this.

Since commit 6cf3f41e6c0 (bonding, net: Move last_rx update into bonding
recv logic), network drivers dont need to update last_rx themselves,
unless they use this field to implement a timeout.

Not updating last_rx helps not dirtying a cache line, improving
performance in SMP.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -953,7 +953,14 @@ struct net_device {
 /*
  * Cache line mostly used on receive path (including eth_type_trans())
  */
-	unsigned long		last_rx;	/* Time of last Rx	*/
+	unsigned long		last_rx;	/* Time of last Rx
+						 * This should not be set in
+						 * drivers, unless really needed,
+						 * because network stack (bonding)
+						 * use it if/when necessary, to
+						 * avoid dirtying this cache line.
+						 */
+
 	/* Interface address info used in eth_type_trans() */
 	unsigned char		*dev_addr;	/* hw address, (before bcast
 						   because most packets are