[For Stable] mm: memcontrol: fix excessive complexity in memory.stat reporting

[Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx>]

In linux stable kernel (tested on 4.14), reading memory.stat in case
of tens of thousands of ghost cgroups pinned by lingering page cache
takes up to 100 ms ~ 700 ms to complete the reading.

Repro steps (tested on 4.14 kernel):

$ cat /tmp/make_zombies

mkdir /tmp/fs
mount -t tmpfs nodev /tmp/fs
for i in {1..10000}; do
   mkdir /sys/fs/cgroup/memory/z$i
   (echo $BASHPID >> /sys/fs/cgroup/memory/z$i/cgroup.procs && echo $i
> /tmp/fs/$i)
 done

# establish baseline
$ perf stat -r3 cat /sys/fs/cgroup/memory/memory.stat > /dev/null
0.011642670 seconds time elapsed

$ bash /tmp/make_zombies
$ perf stat -r3 cat /sys/fs/cgroup/memory/memory.stat > /dev/null
0.134939281 seconds time elapsed

$ rmdir /sys/fs/cgroup/memory/z*
$ perf stat -r3 cat /sys/fs/cgroup/memory/memory.stat > /dev/null
0.135323145 seconds time elapsed
# even after rmdir we have zombies, so still slow.

The fix is already present in linux master (since 4.16) by following commits:

c9019e9bf42e66d028d70d2da6206cad4dd9250d mm: memcontrol: eliminate raw
access to stat and event counters
284542656e22c43fdada8c8cc0ca9ede8453eed7  mm: memcontrol: implement
lruvec stat functions on top of each other
a983b5ebee57209c99f68c8327072f25e0e6e3da  mm: memcontrol: fix
excessive complexity in memory.stat reporting
c3cc39118c3610eb6ab4711bc624af7fc48a35fe  mm: memcontrol: fix
NR_WRITEBACK leak in memcg and system stats
e27be240df53f1a20c659168e722b5d9f16cc7f4  mm: memcg: make sure
memory.events is uptodate when waking pollers

I would like to request cherry-picking the above commits to
linux-stable branch - 4.14.

Thanks,
Vaibhav