controllers: Fix out-of-date README and cgroup mount tests

[fguan322]

Fix: #367

In README file, KConfig "CONFIG_CGROUP_CPUACCT=y", "CONFIG_CGROUP_NET_PRIO=y", "CONFIG_CGROUP_PIDS=y" have been added, which don't exist in early kernel versions but are needed for running test cases in recent kernel versions.

In cgroup_regression_test.sh, the code was out of date. Test 3 and 5 have been modified. For test 3, subsystems cpu and cpuacct have been mounted to the same cgroup tree by systemd, which could be found by "mount | grep cgroup" in terminal (i.e. "cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)"). Therefore, to re-use the cpu subsystem cgroup tree, we should mount "cpu,cpuacct" together to a new directory. For test 5, subsystem1 (rdma on Ubuntu 17.10) and subsystem2 (pids) have been mounted to different cgroup trees. Thus, to re-use these cgroup trees, they should be mounted separately, and if mounted to the same cgroup tree, it should fail.

Signed-off-by: Fang Guan [email protected]