mptcp: support SYSCTL only if enabled

Since the introduction of the sysctl support in MPTCP with
commit 784325e9 ("mptcp: new sysctl to control the activation per NS"),
we don't check CONFIG_SYSCTL.

Until now, that was not an issue: the register and unregister functions
were replaced by NO-OP one if SYSCTL was not enabled in the config. The
only thing we could have avoid is not to reserve memory for the table
but that's for the moment only a small table per net-ns.

But the following commit is going to use SYSCTL_ZERO and SYSCTL_ONE
which are not be defined if SYSCTL is not enabled in the config. This
causes 'undefined reference' errors from the linker.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/ctrl.c

@@ -4,7 +4,9 @@
  * Copyright (c) 2019, Tessares SA.
  */
 
+#ifdef CONFIG_SYSCTL
 #include <linux/sysctl.h>
+#endif
 
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
@@ -15,7 +17,9 @@
 
 static int mptcp_pernet_id;
 struct mptcp_pernet {
+#ifdef CONFIG_SYSCTL
 	struct ctl_table_header *ctl_table_hdr;
+#endif
 
 	int mptcp_enabled;
 	unsigned int add_addr_timeout;
@@ -36,6 +40,13 @@ unsigned int mptcp_get_add_addr_timeout(struct net *net)
 	return mptcp_get_pernet(net)->add_addr_timeout;
 }
 
+static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
+{
+	pernet->mptcp_enabled = 1;
+	pernet->add_addr_timeout = TCP_RTO_MAX;
+}
+
+#ifdef CONFIG_SYSCTL
 static struct ctl_table mptcp_sysctl_table[] = {
 	{
 		.procname = "enabled",
@@ -55,12 +66,6 @@ static struct ctl_table mptcp_sysctl_table[] = {
 	{}
 };
 
-static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
-{
-	pernet->mptcp_enabled = 1;
-	pernet->add_addr_timeout = TCP_RTO_MAX;
-}
-
 static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
 {
 	struct ctl_table_header *hdr;
@@ -100,6 +105,17 @@ static void mptcp_pernet_del_table(struct mptcp_pernet *pernet)
 	kfree(table);
 }
 
+#else
+
+static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
+{
+	return 0;
+}
+
+static void mptcp_pernet_del_table(struct mptcp_pernet *pernet) {}
+
+#endif /* CONFIG_SYSCTL */
+
 static int __net_init mptcp_net_init(struct net *net)
 {
 	struct mptcp_pernet *pernet = mptcp_get_pernet(net);