sctp: allow users to set netns ecn flag with sysctl

sysctl net.sctp.ecn_enable is added in this patch. It will allow
users to change the default sctp ecn flag, net.sctp.ecn_enable.

This feature was also required on this thread:

  http://lkml.iu.edu/hypermail/linux/kernel/0812.1/01858.htmlSigned-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/sysctl.c

@@ -277,6 +277,13 @@ static struct ctl_table sctp_net_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "ecn_enable",
+		.data		= &init_net.sctp.ecn_enable,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
 	{
 		.procname	= "addr_scope_policy",
 		.data		= &init_net.sctp.scope_policy,