selftests/damon/_damon_sysfs.py: fix memcg_path assignment

Patch series "selftests/damon: fix memcg_path staging handling", v5.

Fix a bug in _damon_sysfs.py for damos_filter memcg_path setup, and add
a test case of it in sysfs.py.


This patch (of 2):

DamosFilter stores memcg_path for sysfs staging, but the constructor
assigns it with a trailing comma and therefore turns it into a tuple.

Fix the assignment so memcg_path is stored as the intended string.  This
makes memcg filter staging and follow-up validation use the written path
correctly.

Link: https://lore.kernel.org/20260629144812.134159-1-sj@kernel.org
Link: https://lore.kernel.org/464AE12D4BC6B6F4+20260601090519.240482-1-niecheng1@uniontech.com
Link: https://lore.kernel.org/20260629144812.134159-2-sj@kernel.org
Signed-off-by: Cheng Nie <niecheng1@uniontech.com>
Signed-off-by: SJ Park <sj@kernel.org>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/_damon_sysfs.py

@@ -271,7 +271,7 @@ class DamosFilter:
         self.type_ = type_
         self.matching = matching
         self.allow = allow
-        self.memcg_path = memcg_path,
+        self.memcg_path = memcg_path
         self.addr_start = addr_start
         self.addr_end = addr_end
         self.target_idx = target_idx