cgroup: separate out cgroup_base_files[] handling out of cgroup_populate/clear_dir()

cgroup: separate out cgroup_base_files[] handling out of cgroup_populate/clear_dir()

cgroup_populate/clear_dir() currently take @base_files and adds and
removes, respectively, cgroup_base_files[] to the directory.  File
additions and removals are being reorganized for proper error handling
and more dynamic handling for the unified hierarchy, and mixing base
and subsys file handling into the same functions gets a bit confusing.

This patch moves base file handling out of cgroup_populate/clear_dir()
into their users - cgroup_mount(), cgroup_create() and
cgroup_destroy_locked().

Note that this changes the behavior of base file removal.  If
@base_files is %true, cgroup_clear_dir() used to delete files
regardless of cftype until there's no files left.  Now, only files
with matching cfts are removed.  As files can only be created by the
base or registered cftypes, this shouldn't result in any behavior
difference.

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