cgroup: refactor mount path and clearly distinguish v1 and v2 paths

While sharing some mechanisms, the mount paths of v1 and v2 are
substantially different.  Their implementations were mixed in
cgroup_mount().  This patch splits them out so that they're easier to
follow and organize.

This patch causes one functional change - the WARN_ON(new_sb) gets
lost.  This is because the actual mounting gets moved to
cgroup_do_mount() and thus @new_sb is no longer accessible by default
to cgroup1_mount().  While we can add it as an explicit out parameter
to cgroup_do_mount(), this part of code hasn't changed and the warning
hasn't triggered for quite a while.  Dropping it should be fine.

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