[TCP]: Add tcp_available_congestion_control sysctl.

Create /proc/sys/net/ipv4/tcp_available_congestion_control
that reflects currently available TCP choices.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index d98562f1..28a4827 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -426,6 +426,7 @@
 	NET_CIPSOV4_CACHE_BUCKET_SIZE=119,
 	NET_CIPSOV4_RBM_OPTFMT=120,
 	NET_CIPSOV4_RBM_STRICTVALID=121,
+	NET_TCP_AVAIL_CONG_CONTROL=122,
 };
 
 enum {