tools: add memcg_shrinker.py

tools: add memcg_shrinker.py

Add a simple tool which prints a sorted list of shrinker lists in the
following format: (number of objects, shrinker name, cgroup).

Example:
  $ ./memcg_shrinker.py -n 10
  2090     sb-sysfs-26          /sys/fs/cgroup/system.slice
  1809     sb-sysfs-26          /sys/fs/cgroup/system.slice/systemd-udevd.service
  1044     sb-btrfs:vda2-24     /sys/fs/cgroup/system.slice/system-dbus\x2d:1.3\...
  861      sb-btrfs:vda2-24     /sys/fs/cgroup/system.slice/system-dbus\x2d:1.3\...
  804      sb-btrfs:vda2-24     /sys/fs/cgroup/system.slice
  643      sb-btrfs:vda2-24     /sys/fs/cgroup/system.slice/firewalld.service
  616      sb-cgroup2-30        /sys/fs/cgroup/init.scope
  275      sb-sysfs-26          /
  238      sb-proc-25           /sys/fs/cgroup/system.slice/systemd-journald.service
  225      sb-proc-25           /sys/fs/cgroup/system.slice/abrtd.service

Link: https://lkml.kernel.org/r/20220601032227.4076670-6-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>