selftests: cgroup: refactor proactive reclaim code to reclaim_until()

Refactor the code that drives writing to memory.reclaim (retrying, error
handling, etc) from test_memcg_reclaim() to a helper called
reclaim_until(), which proactively reclaims from a memcg until its usage
reaches a certain value.

While we are at it, refactor and simplify the reclaim loop.

This will be used in a following patch in another test.

Link: https://lkml.kernel.org/r/20221202031512.1365483-3-yosryahmed@google.com


Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Chris Down <chris@chrisdown.name>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vasily Averin <vasily.averin@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>