o disable ipv6 for newly-created net devices to avoid unaccountable pkt counting
   o explicitly enable ip_forward in router namespace
   o proper cleanup in case of validation failure
   o in test_brb, use different tc_index to differentiate the packet source (pem or router_ns)
   o change bpf program return value to 1 so that the packet will be dropped instead of being
     passed up to the stack
Signed-off-by: Yonghong Song <yhs@plumgrid.com>