selftests/bpf: Correct the check of join cgroup

Use ASSERT_OK_FD to check the return value of join cgroup,
or else this test will pass even if the fd < 0. ASSERT_OK_FD
can print the error message to the console.

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/all/6d62bd77-6733-40c7-b240-a1aeff55566c@linux.dev/
Link: https://patch.msgid.link/20250204051154.57655-1-kerneljasonxing@gmail.com