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

This reverts commit ad44e05f which is
commit 07fd5b6cdf3cc30bfde8fe0f644771688be04447 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