selftests/damon/_damon_sysfs: implement kdamonds stop function

Implement DAMON stop function on the test-purpose DAMON sysfs interface
wrapper Python module, _damon_sysfs.py.  This feature will be used by
future DAMON tests that need to start/stop DAMON multiple times.

Link: https://lkml.kernel.org/r/20240625180538.73134-6-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/_damon_sysfs.py

@@ -406,6 +406,10 @@ class Kdamond:
         err = write_file(os.path.join(self.sysfs_dir(), 'state'), 'on')
         return err
 
+    def stop(self):
+        err = write_file(os.path.join(self.sysfs_dir(), 'state'), 'off')
+        return err
+
     def update_schemes_tried_regions(self):
         err = write_file(os.path.join(self.sysfs_dir(), 'state'),
                          'update_schemes_tried_regions')
@@ -511,3 +515,10 @@ class Kdamonds:
             if err is not None:
                 return err
         return None
+
+    def stop(self):
+        for kdamond in self.kdamonds:
+            err = kdamond.stop()
+            if err is not None:
+                return err
+        return None