[NET]: Bring comments in loopback.c uptodate.

A hint as to why it is safe to use per cpu variables,
and note that we actually can have multiple instances
of the loopback device now.
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/loopback.c

@@ -154,6 +154,7 @@ static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
 #endif
 	dev->last_rx = jiffies;
 
+	/* it's OK to use per_cpu_ptr() because BHs are off */
 	pcpu_lstats = netdev_priv(dev);
 	lb_stats = per_cpu_ptr(pcpu_lstats, smp_processor_id());
 	lb_stats->bytes += skb->len;
@@ -221,7 +222,8 @@ static void loopback_dev_free(struct net_device *dev)
 }
 
 /*
- * The loopback device is special. There is only one instance.
+ * The loopback device is special. There is only one instance
+ * per network namespace.
  */
 static void loopback_setup(struct net_device *dev)
 {