net: usb: lan78xx: fix TX byte statistics for small packets

Account for hardware auto-padding in TX byte counters to reflect actual
wire traffic.

The LAN7850 hardware automatically pads undersized frames to the minimum
Ethernet frame length (ETH_ZLEN, 60 bytes). However, the driver tracks
the network statistics based on the unpadded socket buffer length. This
results in the tx_bytes counter under-reporting the actual physical
bytes placed on the Ethernet wire for small packets (like short ARP or
ICMP requests).

Use max_t() to ensure the transmission statistics accurately account for
the hardware-generated padding.

Fixes: d383216 ("lan78xx: Introduce Tx URB processing improvements")
Cc: [email protected]
Signed-off-by: Oleksij Rempel <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: olerem

drivers/net/usb/lan78xx.c

@@ -4178,7 +4178,7 @@ static struct skb_data *lan78xx_tx_buf_fill(struct lan78xx_net *dev,
 		}
 
 		tx_data += len;
-		entry->length += len;
+		entry->length += max_t(unsigned int, len, ETH_ZLEN);
 		entry->num_of_packet += skb_shinfo(skb)->gso_segs ?: 1;
 
 		dev_kfree_skb_any(skb);