net: fix sysctl_fb_tunnels_only_for_init_net link error

The new variable is only available when CONFIG_SYSCTL is enabled,
otherwise we get a link error:

net/ipv4/ip_tunnel.o: In function `ip_tunnel_init_net':
ip_tunnel.c:(.text+0x278b): undefined reference to `sysctl_fb_tunnels_only_for_init_net'
net/ipv6/sit.o: In function `sit_init_net':
sit.c:(.init.text+0x4c): undefined reference to `sysctl_fb_tunnels_only_for_init_net'
net/ipv6/ip6_tunnel.o: In function `ip6_tnl_init_net':
ip6_tunnel.c:(.init.text+0x39): undefined reference to `sysctl_fb_tunnels_only_for_init_net'

This adds an extra condition, keeping the traditional behavior when
CONFIG_SYSCTL is disabled.

Fixes: 79134e6c ("net: do not create fallback tunnels for non-default namespaces")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>