tcp: tcp_fragment() should not assume rtx skbs

While stress testing MTU probing, we had crashes in list_del() that we root-caused
to the fact that tcp_fragment() is unconditionally inserting the freshly allocated
skb into tsorted_sent_queue list.

But this list is supposed to contain skbs that were sent.
This was mostly harmless until MTU probing was enabled.

Fortunately we can use the tcp_queue enum added later (but in same linux version)
for rtx-rb-tree to fix the bug.

Fixes: e2080072 ("tcp: new list for sent but unacked skbs for RACK recovery")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Priyaranjan Jha <priyarjha@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1395,7 +1395,8 @@ int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
 	/* Link BUFF into the send queue. */
 	__skb_header_release(buff);
 	tcp_insert_write_queue_after(skb, buff, sk, tcp_queue);
-	list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);
+	if (tcp_queue == TCP_FRAG_IN_RTX_QUEUE)
+		list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);
 
 	return 0;
 }