cgroup: introduce cgroup_subsys->legacy_name

This allows cgroup subsystems to use a different name on the unified
hierarchy.  cgroup_subsys->name is used on the unified hierarchy,
->legacy_name elsewhere.  If ->legacy_name is not explicitly set, it's
automatically set to ->name and the userland visible behavior remains
unchanged.

v2: Make parse_cgroupfs_options() only consider ->legacy_name as mount
    options are used only on legacy hierarchies.  Suggested by Li
    Zefan.

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