r8169: Fix receive buffer length when MTU is between 1515 and 1536 (Was: r8169: fix driver drop incoming packets >= 1515 ...)

In r8169 driver MTU is used to calculate receive buffer size. Receive buffer size is used to configure hardware incoming packet filter.

For jumbo frames:
Receive buffer size = Max frame size = MTU + 14 (ethernet header) + 4 (vlan header) + 4 (ethernet checksum) = MTU + 22

Bug:
driver for all MTU up to 1536 use receive buffer size 1536

As you can see from formula, this mean all IP packets > 1536 - 22 (for vlan tagged, 1536 - 18 for not tagged) are dropped by hardware filter.

Example:

host_good> ifconfig eth0 mtu 1536
host_r8169> ifconfig eth0 mtu 1536
host_good> ping host_r8169
Ok
host_good> ping -s 1500 host_r8169
Fail
host_good> ifconfig eth0 mtu 7000
host_r8169> ifconfig eth0 mtu 7000
host_good> ping -s 1500 host_r8169
Ok

Bonus: got rid of magic number 8

---

@@ -2229,9 +2229,9 @@ static void __devexit rtl8169_remove_one
 static void rtl8169_set_rxbufsize(struct rtl8169_private *tp,
 				  struct net_device *dev)
 {
-	unsigned int mtu = dev->mtu;
+	unsigned int max_frame = dev->mtu + VLAN_ETH_HLEN + ETH_FCS_LEN;
 
-	tp->rx_buf_sz = (mtu > RX_BUF_SIZE) ? mtu + ETH_HLEN + 8 : RX_BUF_SIZE;
+	tp->rx_buf_sz = (max_frame > RX_BUF_SIZE) ? max_frame : RX_BUF_SIZE;
 }
 
 static int rtl8169_open(struct net_device *dev)

Applied, thank you.