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

This reverts commit 7657e395 which is
commit 07fd5b6c upstream.

It breaks the Android kernel ABI and is not needed for Android devices,
so it is safe to revert for now.  If it is determined that it is needed
in the future, it can be brought back in an abi-preserving way.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I13460fd4409b5bb84a0ae95c790ac604fc1ce7be