md: delete mddev kobj before deleting gendisk kobj

In sync del gendisk path, it deletes gendisk first and the directory
/sys/block/md is removed. Then it releases mddev kobj in a delayed work.
If we enable debug log in sysfs_remove_group, we can see the debug log
'sysfs group bitmap not found for kobject md'. It's the reason that the
parent kobj has been deleted, so it can't find parent directory.

In creating path, it allocs gendisk first, then adds mddev kobj. So it
should delete mddev kobj before deleting gendisk.

Before commit 9e59d609763f ("md: call del_gendisk in control path"), it
releases mddev kobj first. If the kobj hasn't been deleted, it does clean
job and deletes the kobj. Then it calls del_gendisk and releases gendisk
kobj. So it doesn't need to call kobject_del to delete mddev kobj. After
this patch, in sync del gendisk path, the sequence changes. So it needs
to call kobject_del to delete mddev kobj.

After this patch, the sequence is:
1. kobject del mddev kobj
2. del_gendisk deletes gendisk kobj
3. mddev_delayed_delete releases mddev kobj
4. md_kobj_release releases gendisk kobj

Link: https://lore.kernel.org/linux-raid/20250928012424.61370-1-xni@redhat.com
Fixes: 9e59d609763f ("md: call del_gendisk in control path")
Signed-off-by: Xiao Ni <xni@redhat.com>
Reviewed-by: Li Nan <linan122@huawei.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>