selftests/bpf: Fix umount cgroup2 error in test_sockmap

This patch fixes the following "umount cgroup2" error in test_sockmap.c:

 (cgroup_helpers.c:353: errno: Device or resource busy) umount cgroup2

Cgroup fd cg_fd should be closed before cleanup_cgroup_environment().

Fixes: 13a5f3ff ("bpf: Selftests, sockmap test prog run without setting cgroup")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/0399983bde729708773416b8488bac2cd5e022b8.1712639568.git.tanggeliang@kylinos.cnSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/test_sockmap.c

@@ -2104,9 +2104,9 @@ int main(int argc, char **argv)
 		free(options.whitelist);
 	if (options.blacklist)
 		free(options.blacklist);
+	close(cg_fd);
 	if (cg_created)
 		cleanup_cgroup_environment();
-	close(cg_fd);
 	return err;
 }