tcp: speedup tcp_fixup_rcvbuf()

tcp_fixup_rcvbuf() contains a loop to estimate initial socket
rcv space needed for a given mss. With large MTU (like 64K on lo),
we can loop ~500 times and consume a lot of cpu cycles.

perf top of 200 concurrent netperf -t TCP_CRR

5.62%  netperf  [kernel.kallsyms]  [k] tcp_init_buffer_space
1.71%  netperf  [kernel.kallsyms]  [k] _raw_spin_lock
1.55%  netperf  [kernel.kallsyms]  [k] kmem_cache_free
1.51%  netperf  [kernel.kallsyms]  [k] tcp_transmit_skb
1.50%  netperf  [kernel.kallsyms]  [k] tcp_ack

Lets use a 100% factor, and remove the loop.

100% is needed anyway for tcp_adv_win_scale=1
default value, and is also the maximum factor.

Refs: commit b49960a
      ("tcp: change tcp_adv_win_scale and tcp_rmem[2]")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -360,9 +360,7 @@ static void tcp_fixup_rcvbuf(struct sock *sk)
 	if (mss > 1460)
 		icwnd = max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);
 
-	rcvmem = SKB_TRUESIZE(mss + MAX_TCP_HEADER);
-	while (tcp_win_from_space(rcvmem) < mss)
-		rcvmem += 128;
+	rcvmem = 2 * SKB_TRUESIZE(mss + MAX_TCP_HEADER);
 
 	rcvmem *= icwnd;