kselftests: memcg: speed up the memory.high test

After commit 0e4b01df86 ("mm, memcg: throttle allocators when failing
reclaim over memory.high") allocating memory over memory.high became very
time consuming.  But it's exactly what the memory.high test from cgroup
kselftests is doing: it tries to allocate 100M with 30M memory.high value.
It takes forever to complete.

In order to keep it passing (or failing) in a reasonable amount of time
let's try to allocate only a little over 30M: 31M to be precise.

With this change test_memcontrol finishes in a reasonable amount of
time:
  $ time ./test_memcontrol
  ok 1 test_memcg_subtree_control
  ok 2 test_memcg_current
  ok 3 test_memcg_min
  ok 4 test_memcg_low
  ok 5 test_memcg_high
  ok 6 test_memcg_max
  ok 7 test_memcg_oom_events
  ok 8 test_memcg_swap_max
  ok 9 test_memcg_sock
  ok 10 test_memcg_oom_group_leaf_events
  ok 11 test_memcg_oom_group_parent_events
  ok 12 test_memcg_oom_group_score_events

  real	0m2.273s
  user	0m0.064s
  sys	0m0.739s

Link: https://lkml.kernel.org/r/20220415000133.3955987-3-roman.gushchin@linux.dev
Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: David Vernet <void@manifault.com>
Cc: Chris Down <chris@chrisdown.name>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/cgroup/test_memcontrol.c

@@ -607,7 +607,7 @@ static int test_memcg_high(const char *root)
 	if (cg_write(memcg, "memory.high", "30M"))
 		goto cleanup;
 
-	if (cg_run(memcg, alloc_anon, (void *)MB(100)))
+	if (cg_run(memcg, alloc_anon, (void *)MB(31)))
 		goto cleanup;
 
 	if (!cg_run(memcg, alloc_pagecache_50M_check, NULL))