cgroup: make CFTYPE_ONLY_ON_DFL and CFTYPE_NO_ internal to cgroup core

cgroup now distinguishes cftypes for the default and legacy
hierarchies more explicitly by using separate arrays and
CFTYPE_ONLY_ON_DFL and CFTYPE_INSANE should be and are used only
inside cgroup core proper.  Let's make it clear that the flags are
internal by prefixing them with double underscores.

CFTYPE_INSANE is renamed to __CFTYPE_NOT_ON_DFL for consistency.  The
two flags are also collected and assigned bits >= 16 so that they
aren't mixed with the published flags.

v2: Convert the extra ones in cgroup_exit_cftypes() which are added by
    revision to the previous patch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>