selftests: forwarding: lib: add netns support for tc rule handle stats get

When run the test in netns, it's not easy to get the tc stats via
tc_rule_handle_stats_get(). With the new netns parameter, we can get
stats from specific netns like

  num=$(tc_rule_handle_stats_get "dev eth0 ingress" 101 ".packets" "-n ns")

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>