Add support for memcg v2 attributes

Some but not all memcg v1 attributes exist in the v2 hierarchy. The
following information comes from Shakeel Butt and Kamil Yurtsever,
"cgroup v2 migration at Google", Linux Plumbers Conference 2018
( https://lpc.events/event/2/contributions/204/attachments/143/378/LPC2018-cgroup-v2.pdf

 ):

         v1                    v2
-------------------------- ----------
memory.limit_in_bytes      memory.max
memory.soft_limit_in_bytes memory.low

Add support for cgroup attributes that have different names in the v1
and v2 hierarchies. Also add those memcg attributes that are used in
lmkd.

Bug: 213617178
Test: Tested lmkd with v1 and v2 memcg.
Change-Id: Ia5c5f02974f3b783d5cdaad85b33169ced8db55f
Signed-off-by: Bart Van Assche <bvanassche@google.com>