net: fix build without CONFIG_SYSCTL definition

Earlier commit 316cdaa1 ("net: add option to not create fall-back
tunnels in root-ns as well") removed the CONFIG_SYSCTL to enable the
kernel-commandline to work. However, this variable gets defined only
when CONFIG_SYSCTL option is selected.

With this change the behavior would default to creating fall-back
tunnels in all namespaces when CONFIG_SYSCTL is not selected and
the kernel commandline option will be ignored.

Fixes: 316cdaa1 ("net: add option to not create fall-back tunnels in root-ns as well")
Signed-off-by: NMahesh Bandewar <maheshb@google.com>
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Reported-by: Nkernel test robot <lkp@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -634,8 +634,9 @@ extern int sysctl_devconf_inherit_init_net;
  */
 static inline bool net_has_fallback_tunnels(const struct net *net)
 {
-	return (net == &init_net && sysctl_fb_tunnels_only_for_init_net == 1) ||
-	       !sysctl_fb_tunnels_only_for_init_net;
+	return !IS_ENABLED(CONFIG_SYSCTL) ||
+	       !sysctl_fb_tunnels_only_for_init_net ||
+	       (net == &init_net && sysctl_fb_tunnels_only_for_init_net == 1);
 }
 
 static inline int netdev_queue_numa_node_read(const struct netdev_queue *q)