net: bcmgenet: fix skb_len in bcmgenet_xmit_single()

BugLink: https://bugs.launchpad.net/bugs/1878232

commit 7dd39913 upstream.

skb_len needs to be skb_headlen(skb) in bcmgenet_xmit_single().

Fragmented skbs can have only Ethernet + IP + TCP headers (14+20+20=54 bytes)
in the linear buffer, followed by the rest in fragments. Bumping skb_len to
ETH_ZLEN would be incorrect for this case, as it would introduce garbage
between TCP header and the fragment data.

This also works with regular/non-fragmented small packets < ETH_ZLEN bytes.
Successfully tested this on GENETv3 with 42-byte ARP frames.

For testing, I used:
ethtool -K eth0 tx-checksum-ipv4 off
ethtool -K eth0 tx-checksum-ipv6 off
echo 0 > /proc/sys/net/ipv4/tcp_timestamps

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Signed-off-by: Petri Gynther <pgynther@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/ethernet/broadcom/genet/bcmgenet.c

@@ -1346,7 +1346,7 @@ static int bcmgenet_xmit_single(struct net_device *dev,
 
 	tx_cb_ptr->skb = skb;
 
-	skb_len = skb_headlen(skb) < ETH_ZLEN ? ETH_ZLEN : skb_headlen(skb);
+	skb_len = skb_headlen(skb);
 
 	mapping = dma_map_single(kdev, skb->data, skb_len, DMA_TO_DEVICE);
 	ret = dma_mapping_error(kdev, mapping);