selftests/bpf: Add checks in tc_tunnel when entering net namespaces

selftests/bpf: Add checks in tc_tunnel when entering net namespaces

test_tc_tunnel is missing checks on any open_netns. Add those checks
anytime we try to enter a net namespace, and skip the related operations
if we fail. While at it, reduce the number of open_netns/close_netns for
cases involving operations in two distinct namespaces: the test
currently does the following:

  nstoken = open_netns("foo")
  do_operation();
  close(nstoken);
  nstoken = open_netns("bar")
  do_another_operation();
  close(nstoken);

As already stated in reviews for the initial test, we don't need to go
back to the root net namespace to enter a second namespace, so just do:

  ntoken_client = open_netns("foo")
  do_operation();
  nstoken_server = open_netns("bar")
  do_another_operation();
  close(nstoken_server);
  close(nstoken_client);

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20251031-tc_tunnel_improv-v1-2-0ffe44d27eda@bootlin.com