mm/damon/core: warn and fix nr_accesses[_bp] corruption

mm/damon/core: warn and fix nr_accesses[_bp] corruption

Patch series "mm/damon: minor fixups and improvements for code, tests, and
documents".

Yet another batch of miscellaneous DAMON changes.  Fix and improve minor
problems in code, tests and documents.

This patch (of 6):

For a bug such as double aggregation reset[1], ->nr_accesses and/or
->nr_accesses_bp of damon_region could be corrupted.  Such corruption can
make monitoring results pretty inaccurate, so the root causing bug should
be investigated.  Meanwhile, the corruption itself can easily be fixed but
silently fixing it will hide the bug.

Fix the corruption as soon as found, but WARN_ONCE() so that we can be
aware of the existence of the bug while keeping the system running in a
more sane way.

Link: https://lkml.kernel.org/r/20250513002715.40126-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250513002715.40126-2-sj@kernel.org
Link: https://lore.kernel.org/20250302214145.356806-1-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>