[DCCP] CCID2: Initialize ssthresh to infinity

Initialize the slow-start threshold to infinity.  This way, upon connection
initiation, slow-start will be exited only upon a packet loss.  This patch will
allow connections to quickly gain speed.

Signed-off-by: Andrea Bittau <a.bittau@cs.ucl.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>