[TCP]: Set initial_ssthresh default to zero in Cubic and BIC.

Because of the current default of 100, Cubic and BIC perform very
poorly compared to standard Reno.

In the worst case, this change makes Cubic and BIC as aggressive as
Reno.  So this change should be very safe.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_bic.c

@@ -29,7 +29,7 @@ static int fast_convergence = 1;
 static int max_increment = 16;
 static int low_window = 14;
 static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
-static int initial_ssthresh = 100;
+static int initial_ssthresh;
 static int smooth_part = 20;
 
 module_param(fast_convergence, int, 0644);

net/ipv4/tcp_cubic.c

@@ -29,7 +29,7 @@
 static int fast_convergence __read_mostly = 1;
 static int max_increment __read_mostly = 16;
 static int beta __read_mostly = 819;	/* = 819/1024 (BICTCP_BETA_SCALE) */
-static int initial_ssthresh __read_mostly = 100;
+static int initial_ssthresh __read_mostly;
 static int bic_scale __read_mostly = 41;
 static int tcp_friendliness __read_mostly = 1;