tcp: do not set snd_ssthresh in tcp_create_openreq_child()

New sockets get the field set to TCP_INFINITE_SSTHRESH in tcp_init_sock()
In case a socket had this field changed and transitions to TCP_LISTEN
state, tcp_disconnect() also makes sure snd_ssthresh is set to
TCP_INFINITE_SSTHRESH.

So a listener has this field set to TCP_INFINITE_SSTHRESH already.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>