net: refine debug info in skb_checksum_help()

syzbot uses panic_on_warn.

This means that the skb_dump() I added in the blamed commit are
not even called.

Rewrite this so that we get the needed skb dump before syzbot crashes.

Fixes: eeee4b77 ("net: add more debug info in skb_checksum_help()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20231006173355.2254983-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -3292,15 +3292,19 @@ int skb_checksum_help(struct sk_buff *skb)
 
 	offset = skb_checksum_start_offset(skb);
 	ret = -EINVAL;
-	if (WARN_ON_ONCE(offset >= skb_headlen(skb))) {
+	if (unlikely(offset >= skb_headlen(skb))) {
 		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
+		WARN_ONCE(true, "offset (%d) >= skb_headlen() (%u)\n",
+			  offset, skb_headlen(skb));
 		goto out;
 	}
 	csum = skb_checksum(skb, offset, skb->len - offset, 0);
 
 	offset += skb->csum_offset;
-	if (WARN_ON_ONCE(offset + sizeof(__sum16) > skb_headlen(skb))) {
+	if (unlikely(offset + sizeof(__sum16) > skb_headlen(skb))) {
 		DO_ONCE_LITE(skb_dump, KERN_ERR, skb, false);
+		WARN_ONCE(true, "offset+2 (%zu) > skb_headlen() (%u)\n",
+			  offset + sizeof(__sum16), skb_headlen(skb));
 		goto out;
 	}
 	ret = skb_ensure_writable(skb, offset + sizeof(__sum16));