selftests/bpf: test_sockmap, join cgroup in selftest mode

In case of selftest mode, temporary cgroup environment is created but
cgroup is not joined. It causes test failures. Fixed by joining the
cgroup

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: NPrashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_sockmap.c

@@ -1344,6 +1344,11 @@ static int __test_suite(char *bpf_file)
 		return cg_fd;
 	}
 
+	if (join_cgroup(CG_PATH)) {
+		fprintf(stderr, "ERROR: failed to join cgroup\n");
+		return -EINVAL;
+	}
+
 	/* Tests basic commands and APIs with range of iov values */
 	txmsg_start = txmsg_end = 0;
 	err = test_txmsg(cg_fd);