memcg_control: revert memory.use_hierarchy value to 0

Documentation/cgroups/memory.txt states that "Enabling/disabling will
fail if either the cgroup already has other cgroups created below it,
or if the parent cgroup has use_hierarchy enabled."

memcg_control_test.sh sets global memory.use_hierarchy value to 1 before
the execution of its own tests but doesn't revert it back to 0 after
they complete.

It causes failures of memcg_stat_test.sh and memcg_use_hierarchy_test.sh
testcases if runtest/controllers is run twice on the same host.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>