net: xfrm: reject unrepresentable espintcp transport headers

ESP-in-TCP can hand xfrm packets whose transport header offset no longer
fits after the stream parser trims the TCP envelope. The plain transport
header reset truncates that offset and triggers the skb warning path.

Use the careful transport-header helper and drop the skb through the
existing XFRM error path when the offset cannot be represented.

Fixes: e27cca96cd68 ("xfrm: add espintcp (RFC 8229)")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:GPT-5.4
Signed-off-by: Wyatt Feng <wf.kernel.dev@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>