kernel: mtk_eth_soc: fix encapsulation check for GSO fraglist packets

Use skb->encapsulation flag instead of skb_tnl_header_len() to detect
encapsulated packets. Prevents false positives on non-encapsulated packets.

Reported-by: Mason-cw Chang (張哲維) <[email protected]>
Signed-off-by: Felix Fietkau <[email protected]>

target/linux/generic/pending-6.12/732-05-net-ethernet-mtk_eth_soc-add-support-for-sending-fra.patch

@@ -427,7 +427,7 @@ Signed-off-by: Felix Fietkau <[email protected]>
 +	if (!(skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST))
 +		return true;
 +
-+	if (skb_tnl_header_len(skb))
++	if (skb->encapsulation)
 +		return false;
 +
 +	return skb_pagelen(skb) - header_len == skb_shinfo(skb)->gso_size &&