mptcp: sysctl: avail sched: remove write access

'net.mptcp.available_schedulers' sysctl knob is there to list available
schedulers, not to modify this list.

There are then no reasons to give write access to it.

Nothing would have been written anyway, but no errors would have been
returned, which is unexpected.

Fixes: 73c900a ("mptcp: add net.mptcp.available_schedulers")
Cc: [email protected]
Reviewed-by: Mat Martineau <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

net/mptcp/ctrl.c

@@ -228,7 +228,7 @@ static struct ctl_table mptcp_sysctl_table[] = {
 	{
 		.procname = "available_schedulers",
 		.maxlen	= MPTCP_SCHED_BUF_MAX,
-		.mode = 0644,
+		.mode = 0444,
 		.proc_handler = proc_available_schedulers,
 	},
 	{