Testing cgroup2 can be painful with system software automatically
mounting and populating all cgroup controllers in v1 mode. Sometimes
they can be unmounted from rc.local, sometimes even that is too late.

Provide a commandline option to disable certain controllers in v1
mounts, so that they remain available for cgroup2 mounts.

Example use:

cgroup_no_v1=memory,cpu
cgroup_no_v1=all

Disabling will be confirmed at boot-time as such:

[    0.013770] Disabling cpu control group subsystem in v1 mounts
[    0.016004] Disabling memory control group subsystem in v1 mounts

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Tejun Heo <tj@kernel.org>