net: usb: smsc75xx: stop lying about skb->truesize

Some usb drivers try to set small skb->truesize and break
core networking stacks.

In this patch, I removed one of the skb->truesize override.

I also replaced one skb_clone() by an allocation of a fresh
and small skb, to get minimally sized skbs, like we did
in commit 1e2c611 ("net: cdc_ncm: reduce skb truesize
in rx path") and 4ce62d5 ("net: usb: ax88179_178a:
stop lying about skb->truesize")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Link: https://lore.kernel.org/r/20240506142358.3657918-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

drivers/net/usb/smsc75xx.c

@@ -2234,27 +2234,23 @@ static int smsc75xx_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 					rx_cmd_b);
 
 				skb_trim(skb, skb->len - 4); /* remove fcs */
-				skb->truesize = size + sizeof(struct sk_buff);
 
 				return 1;
 			}
 
-			ax_skb = skb_clone(skb, GFP_ATOMIC);
+			/* Use "size - 4" to remove fcs */
+			ax_skb = netdev_alloc_skb_ip_align(dev->net, size - 4);
 			if (unlikely(!ax_skb)) {
 				netdev_warn(dev->net, "Error allocating skb\n");
 				return 0;
 			}
 
-			ax_skb->len = size;
-			ax_skb->data = packet;
-			skb_set_tail_pointer(ax_skb, size);
+			skb_put(ax_skb, size - 4);
+			memcpy(ax_skb->data, packet, size - 4);
 
 			smsc75xx_rx_csum_offload(dev, ax_skb, rx_cmd_a,
 				rx_cmd_b);
 
-			skb_trim(ax_skb, ax_skb->len - 4); /* remove fcs */
-			ax_skb->truesize = size + sizeof(struct sk_buff);
-
 			usbnet_skb_return(dev, ax_skb);
 		}