cgroup/dmem: avoid rcu warning when unregister region

cgroup/dmem: avoid rcu warning when unregister region

A warnning was detected:

 WARNING: suspicious RCU usage
 6.19.0-rc7-next-20260129+ #1101 Tainted: G           O
 kernel/cgroup/dmem.c:456 suspicious rcu_dereference_check() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by insmod/532:
  #0: ffffffff85e78b38 (dmemcg_lock){+.+.}-dmem_cgroup_unregister_region+

 stack backtrace:
 CPU: 2 UID: 0 PID: 532 Comm: insmod Tainted: 6.19.0-rc7-next-
 Tainted: [O]=OOT_MODULE
 Call Trace:
  <TASK>
  dump_stack_lvl+0xb0/0xd0
  lockdep_rcu_suspicious+0x151/0x1c0
  dmem_cgroup_unregister_region+0x1e2/0x380
  ? __pfx_dmem_test_init+0x10/0x10 [dmem_uaf]
  dmem_test_init+0x65/0xff0 [dmem_uaf]
  do_one_initcall+0xbb/0x3a0

The macro list_for_each_rcu() must be used within an RCU read-side critical
section (between rcu_read_lock() and rcu_read_unlock()). Using it outside
that context, as seen in dmem_cgroup_unregister_region(), triggers the
lockdep warning because the RCU protection is not guaranteed.

Replace list_for_each_rcu() with list_for_each_entry_safe(), which is
appropriate for traversal under spinlock protection where nodes may be
deleted.

Fixes: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup")
Cc: stable@vger.kernel.org # v6.14+
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>