block: fix use-after-free in disk_part_iter_next

stable inclusion
from linux-4.19.168
commit 3d13ebbd06697e2fb180a61bbff1ff244a9513e4

--------------------------------

commit aebf5db9 upstream.

Make sure that bdgrab() is done on the 'block_device' instance before
referring to it for avoiding use-after-free.

Cc: <stable@vger.kernel.org>
Reported-by:  <syzbot+825f0f9657d4e528046e@syzkaller.appspotmail.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>