tcp: remove obsolete check in __tcp_retransmit_skb()

TSQ provides a nice way to avoid bufferbloat on individual socket,
including retransmit packets. We can get rid of the old
heuristic:

	/* Do not sent more than we queued. 1/4 is reserved for possible
	 * copying overhead: fragmentation, tunneling, mangling etc.
	 */
	if (refcount_read(&sk->sk_wmem_alloc) >
	    min_t(u32, sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2),
		  sk->sk_sndbuf))
		return -EAGAIN;

This heuristic was giving false positives according to Jakub,
whenever TX completions are delayed above RTT. (Ack packets
are processed by TCP stack before clones are orphaned/freed)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -3151,14 +3151,6 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	if (icsk->icsk_mtup.probe_size)
 		icsk->icsk_mtup.probe_size = 0;
 
-	/* Do not sent more than we queued. 1/4 is reserved for possible
-	 * copying overhead: fragmentation, tunneling, mangling etc.
-	 */
-	if (refcount_read(&sk->sk_wmem_alloc) >
-	    min_t(u32, sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2),
-		  sk->sk_sndbuf))
-		return -EAGAIN;
-
 	if (skb_still_in_host_queue(sk, skb))
 		return -EBUSY;