cgroup/dmem: fix NULL pointer dereference when setting max

cgroup/dmem: fix NULL pointer dereference when setting max

An issue was triggered:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0000 [#1] SMP NOPTI
 CPU: 15 UID: 0 PID: 658 Comm: bash Tainted: 6.19.0-rc6-next-2026012
 Tainted: [O]=OOT_MODULE
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
 RIP: 0010:strcmp+0x10/0x30
 RSP: 0018:ffffc900017f7dc0 EFLAGS: 00000246
 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff888107cd4358
 RDX: 0000000019f73907 RSI: ffffffff82cc381a RDI: 0000000000000000
 RBP: ffff8881016bef0d R08: 000000006c0e7145 R09: 0000000056c0e714
 R10: 0000000000000001 R11: ffff888107cd4358 R12: 0007ffffffffffff
 R13: ffff888101399200 R14: ffff888100fcb360 R15: 0007ffffffffffff
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000000105c79000 CR4: 00000000000006f0
 Call Trace:
  <TASK>
  dmemcg_limit_write.constprop.0+0x16d/0x390
  ? __pfx_set_resource_max+0x10/0x10
  kernfs_fop_write_iter+0x14e/0x200
  vfs_write+0x367/0x510
  ksys_write+0x66/0xe0
  do_syscall_64+0x6b/0x390
  entry_SYSCALL_64_after_hwframe+0x76/0x7e
 RIP: 0033:0x7f42697e1887

It was trriggered setting max without limitation, the command is like:
"echo test/region0 > dmem.max". To fix this issue, add check whether
options is valid after parsing the region_name.

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>