selftests/damon/sysfs.py: pause DAMON before dumping status

selftests/damon/sysfs.py: pause DAMON before dumping status

The sysfs.py test commits DAMON parameters, dump the internal DAMON state,
and show if the parameters are committed as expected using the dumped
state.  While the dumping is ongoing, DAMON is alive.  It can make
internal changes including addition and removal of regions.  It can
therefore make a race that can result in false test results.  Pause DAMON
execution during the state dumping to avoid such races.

Link: https://lore.kernel.org/20260427151231.113429-11-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>