Merge branch 'for-4.13-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup fixes from Tejun Heo:
 "Several cgroup bug fixes.

   - cgroup core was calling a migration callback on empty migrations,
     which could make cpuset crash.

   - There was a very subtle bug where the controller interface files
     aren't created directly when cgroup2 is mounted. Because later
     operations create them, this bug didn't get noticed earlier.

   - Failed writes to cgroup.subtree_control were incorrectly returning
     zero"

* 'for-4.13-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  cgroup: fix error return value from cgroup_subtree_control()
  cgroup: create dfl_root files on subsys registration
  cgroup: don't call migration methods if there are no tasks to migrate