md: delete md_redundancy_group when array is becoming inactive

'md_redundancy_group' are created in md_run() and deleted in del_gendisk(),
but these are not paired. Writing inactive/active to sysfs array_state can
trigger md_run() multiple times without del_gendisk(), leading to
duplicate creation as below:

 sysfs: cannot create duplicate filename '/devices/virtual/block/md0/md/sync_action'
 Call Trace:
  dump_stack_lvl+0x9f/0x120
  dump_stack+0x14/0x20
  sysfs_warn_dup+0x96/0xc0
  sysfs_add_file_mode_ns+0x19c/0x1b0
  internal_create_group+0x213/0x830
  sysfs_create_group+0x17/0x20
  md_run+0x856/0xe60
  ? __x64_sys_openat+0x23/0x30
  do_md_run+0x26/0x1d0
  array_state_store+0x559/0x760
  md_attr_store+0xc9/0x1e0
  sysfs_kf_write+0x6f/0xa0
  kernfs_fop_write_iter+0x141/0x2a0
  vfs_write+0x1fc/0x5a0
  ksys_write+0x79/0x180
  __x64_sys_write+0x1d/0x30
  x64_sys_call+0x2818/0x2880
  do_syscall_64+0xa9/0x580
  entry_SYSCALL_64_after_hwframe+0x4b/0x53
 md: cannot register extra attributes for md0

Creation of it depends on 'pers', its lifecycle cannot be aligned with
gendisk. So fix this issue by triggering 'md_redundancy_group' deletion
when the array is becoming inactive.

Link: https://lore.kernel.org/linux-raid/20251103125757.1405796-2-linan666@huaweicloud.com
Fixes: 790abe4d77 ("md: remove/add redundancy group only in level change")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>

drivers/md/md.c

@@ -6879,6 +6879,10 @@ static int do_md_stop(struct mddev *mddev, int mode)
 		if (!md_is_rdwr(mddev))
 			set_disk_ro(disk, 0);
 
+		if (mode == 2 && mddev->pers->sync_request &&
+		    mddev->to_remove == NULL)
+			mddev->to_remove = &md_redundancy_group;
+
 		__md_stop_writes(mddev);
 		__md_stop(mddev);