net: tcp: fix crashes trying to free half-baked MTU probes

tcp_stream_alloc_skb() initializes the skb to use tcp_tsorted_anchor
which is a union with the destructor. We need to clean that
TCP-iness up before freeing.

Fixes: 73601329 ("tcp: let tcp_mtu_probe() build headless packets")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20231010173651.3990234-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>