r8169: de-obfuscate modulo arithmetic

The former style suggests a modulo arithmetic misuse but
the expression should never be < 0. Even if it does, the
driver will simply loop longer than expected (not that
the remaining parts of the system will necessarily
appreciate it...).

Let's warn the user when something goes wrong and try
to go over it.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>

drivers/net/r8169.c

@@ -2040,10 +2040,12 @@ static u32 rtl8169_rx_fill(struct rtl8169_private *tp, struct net_device *dev,
 {
 	u32 cur;
 
-	for (cur = start; end - cur > 0; cur++) {
+	for (cur = start; end - cur != 0; cur++) {
 		struct sk_buff *skb;
 		unsigned int i = cur % NUM_RX_DESC;
 
+		WARN_ON((s32)(end - cur) < 0);
+
 		if (tp->Rx_skbuff[i])
 			continue;