mm/page_owner: fix memory leak in page_owner_stack_fops->release()

mm/page_owner: fix memory leak in page_owner_stack_fops->release()

The page_owner_stack_fops->open() callback invokes seq_open_private(),
therefore its corresponding ->release() callback must call
seq_release_private().  Otherwise it will cause a memory leak of struct
stack_print_ctx.

Link: https://lkml.kernel.org/r/20251219074232.136482-1-ranxiaokai627@163.com
Fixes: 765973a09803 ("mm,page_owner: display all stacks and their count")
Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Marco Elver <elver@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>