selftests/damon/damon_nr_regions: test online-tuned max_nr_regions

User could update max_nr_regions parameter while DAMON is running to a
value that smaller than the current number of regions that DAMON is
seeing.  Such update could be done for reducing the monitoring overhead. 
In the case, DAMON should merge regions aggressively more than normal
situation to ensure the new limit is successfully applied.  Implement a
kselftest to ensure that.

Link: https://lkml.kernel.org/r/20240625180538.73134-9-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>