tcp: fix ssthresh u16 leftover

It was once upon time so that snd_sthresh was a 16-bit quantity.
...That has not been true for long period of time. I run across
some ancient compares which still seem to trust such legacy.
Put all that magic into a single place, I hopefully found all
of them.

Compile tested, though linking of allyesconfig is ridiculous
nowadays it seems.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -2012,7 +2012,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tp->snd_cwnd = 2;
 	icsk->icsk_probes_out = 0;
 	tp->packets_out = 0;
-	tp->snd_ssthresh = 0x7fffffff;
+	tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	tp->snd_cwnd_cnt = 0;
 	tp->bytes_acked = 0;
 	tcp_set_ca_state(sk, TCP_CA_Open);