mm/damon/sysfs-schemes: free old damon_sysfs_scheme_filter->memcg_path on write

memcg_path_store() assigns a newly allocated memory buffer to
filter->memcg_path, without deallocating the previously allocated and
assigned memory buffer.  As a result, users can leak kernel memory by
continuously writing a data to memcg_path DAMOS sysfs file.  Fix the leak
by deallocating the previously set memory buffer.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7ee161f ("mm/damon/sysfs-schemes: implement filter directory")
Signed-off-by: SeongJae Park <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: <[email protected]>		[6.3.x]
Signed-off-by: Andrew Morton <[email protected]>

Author: sjp38

mm/damon/sysfs-schemes.c

@@ -472,6 +472,7 @@ static ssize_t memcg_path_store(struct kobject *kobj,
 		return -ENOMEM;
 
 	strscpy(path, buf, count + 1);
+	kfree(filter->memcg_path);
 	filter->memcg_path = path;
 	return count;
 }