cgroup: replace __DEVEL__sane_behavior with cgroup2 fs type

With major controllers - cpu, memory and io - shaping up for the
unified hierarchy, cgroup2 is about ready to be, gradually, released
into the wild.  Replace __DEVEL__sane_behavior flag which was used to
select the unified hierarchy with a separate filesystem type "cgroup2"
so that unified hierarchy can be mounted as follows.

  mount -t cgroup2 none $MOUNT_POINT

The cgroup2 fs has its own magic number - 0x63677270 ("cgrp").

v2: Assign a different magic number to cgroup2 fs.

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