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

tools/testing/selftests/net/forwarding/lib.sh

@@ -791,8 +791,9 @@ tc_rule_handle_stats_get()
 	local id=$1; shift
 	local handle=$1; shift
 	local selector=${1:-.packets}; shift
+	local netns=${1:-""}; shift
 
-	tc -j -s filter show $id \
+	tc $netns -j -s filter show $id \
 	    | jq ".[] | select(.options.handle == $handle) | \
 		  .options.actions[0].stats$selector"
 }