net: add option to not create fall-back tunnels in root-ns as well

The sysctl that was added  earlier by commit 79134e6ce2 ("net: do
not create fallback tunnels for non-default namespaces") to create
fall-back only in root-ns. This patch enhances that behavior to provide
option not to create fallback tunnels in root-ns as well. Since modules
that create fallback tunnels could be built-in and setting the sysctl
value after booting is pointless, so added a kernel cmdline options to
change this default. The default setting is preseved for backward
compatibility. The kernel command line option of fb_tunnels=initns will
set the sysctl value to 1 and will create fallback tunnels only in initns
while kernel cmdline fb_tunnels=none will set the sysctl value to 2 and
fallback tunnels are skipped in every netns.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Maciej Zenczykowski <maze@google.com>
Cc: Jian Yang <jianyang@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -640,10 +640,14 @@ struct netdev_queue {
 extern int sysctl_fb_tunnels_only_for_init_net;
 extern int sysctl_devconf_inherit_init_net;
 
+/*
+ * sysctl_fb_tunnels_only_for_init_net == 0 : For all netns
+ *                                     == 1 : For initns only
+ *                                     == 2 : For none.
+ */
 static inline bool net_has_fallback_tunnels(const struct net *net)
 {
-	return net == &init_net ||
-	       !IS_ENABLED(CONFIG_SYSCTL) ||
+	return (net == &init_net && sysctl_fb_tunnels_only_for_init_net == 1) ||
 	       !sysctl_fb_tunnels_only_for_init_net;
 }