Revert "cgroup: Use separate src/dst nodes when preloading css_sets for migration"

This reverts commit cec2bbdc which is
commit 07fd5b6c upstream.

It breaks the kernel ABI and is not something that is an issue for
Android devices, so revert it.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I89125948bbf90805d25a446afeab69f6581442bc