cilium: ipsec, support kernel without ipv6 support
cilium: ipsec, support kernel without ipv6 support

If kernel does not have ipv6 support forwarding file will not exist
so do not try to set the file if ipv6 is disabled.

Signed-off-by: John Fastabend <ee461b60978bea1c76b3018c0b89685175b248e2@gmail.com>

tklauser/cilium,cilium/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium-team/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium