selftests/bpf: Fix umount cgroup2 error in test_sockmap

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: 13a5f3ffd202 ("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.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>