selftests/damon/_damon_sysfs: support obsolete_target file

A DAMON sysfs file, namely obsolete_target, has been newly introduced. 
Add a support of that file to _damon_sysfs.py so that DAMON selftests for
the file can be easily written.

Link: https://lkml.kernel.org/r/20251023012535.69625-7-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Bijan Tabatabai <bijan311@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
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@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/_damon_sysfs.py

@@ -475,12 +475,14 @@ class Damos:
 
 class DamonTarget:
     pid = None
+    obsolete = None
     # todo: Support target regions if test is made
     idx = None
     context = None
 
-    def __init__(self, pid):
+    def __init__(self, pid, obsolete=False):
         self.pid = pid
+        self.obsolete = obsolete
 
     def sysfs_dir(self):
         return os.path.join(
@@ -491,8 +493,13 @@ class DamonTarget:
                 os.path.join(self.sysfs_dir(), 'regions', 'nr_regions'), '0')
         if err is not None:
             return err
-        return write_file(
+        err = write_file(
                 os.path.join(self.sysfs_dir(), 'pid_target'), self.pid)
+        if err is not None:
+            return err
+        return write_file(
+                os.path.join(self.sysfs_dir(), 'obsolete_target'),
+                'Y' if self.obsolete else 'N')
 
 class IntervalsGoal:
     access_bp = None