[NET]: Make illegal_highdma more anal

Rather than having illegal_highdma as a macro when HIGHMEM is off, we
can turn it into an inline function that returns zero.  This will catch
callers that give it bad arguments.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1239,7 +1239,6 @@ void netdev_rx_csum_fault(struct net_device *dev)
 EXPORT_SYMBOL(netdev_rx_csum_fault);
 #endif
 
-#ifdef CONFIG_HIGHMEM
 /* Actually, we should eliminate this check as soon as we know, that:
  * 1. IOMMU is present and allows to map all the memory.
  * 2. No high memory really exists on this machine.
@@ -1247,6 +1246,7 @@ EXPORT_SYMBOL(netdev_rx_csum_fault);
 
 static inline int illegal_highdma(struct net_device *dev, struct sk_buff *skb)
 {
+#ifdef CONFIG_HIGHMEM
 	int i;
 
 	if (dev->features & NETIF_F_HIGHDMA)
@@ -1256,11 +1256,9 @@ static inline int illegal_highdma(struct net_device *dev, struct sk_buff *skb)
 		if (PageHighMem(skb_shinfo(skb)->frags[i].page))
 			return 1;
 
+#endif
 	return 0;
 }
-#else
-#define illegal_highdma(dev, skb)	(0)
-#endif
 
 struct dev_gso_cb {
 	void (*destructor)(struct sk_buff *skb);