selftests/bpf: Fix a fd leak in error paths in open_netns

As Martin mentioned in review comment, there is an existing bug that
orig_netns_fd will be leaked in the later "goto fail;" case after
open("/proc/self/ns/net") in open_netns() in network_helpers.c. This
patch adds "close(token->orig_netns_fd);" before "free(token);" to
fix it.

Fixes: a3033884 ("selftests/bpf: Move open_netns() and close_netns() into network_helpers.c")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/a104040b47c3c34c67f3f125cdfdde244a870d3c.1713868264.git.tanggeliang@kylinos.cnSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/network_helpers.c

@@ -478,6 +478,8 @@ struct nstoken *open_netns(const char *name)
 
 	return token;
 fail:
+	if (token->orig_netns_fd != -1)
+		close(token->orig_netns_fd);
 	free(token);
 	return NULL;
 }