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 <[e-mail]>
Signed-off-by: Jason Xing <[e-mail]>
Signed-off-by: Martin KaFai Lau <[e-mail]>
Acked-by: Hou Tao <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Link: https://patch.msgid.link/[e-mail]

tools/testing/selftests/bpf/prog_tests/setget_sockopt.c

···
 void test_setget_sockopt(void)
 {
 	cg_fd = test__join_cgroup(CG_NAME);
-	if (cg_fd < 0)
+	if (!ASSERT_OK_FD(cg_fd, "join cgroup"))
 		return;
 
 	if (create_netns())