controllers/memcg_lib.sh: reset root cgroup's memory.use_hierarchy to 0

In some linux distribution, such as RHEL7U0Beta, memory.use_hierarchy is
set to 1 by systemd default, which will cause some tests failed, such as
memcg_stat_test.sh, memcg_use_hierarchy_test.sh.

According to kernel Documentation/cgroups/memory.txt's description,
the default value should be 0, and some ltp tests are written based on
this assumption, so here we set the root cgroup's memory.use_hierarchy
to 0 before running the test and restore it when finishing the test.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>