block: cleanup partition scanning in register_disk

mainline inclusion
from mainline-v5.10-rc1
commit 9301fe73
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6MRB5
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9301fe734384990ef9a2463cb7aeb3b00bf5dad5



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Use blkdev_get_by_dev instead of open coding it using bdget_disk +
blkdev_get, and split the code to read the partition table into a
separate helper to make it a little more obvious.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Conflict:
 - this patch just factor out a helper, bdget_disk + blkdev_get is still
 used because 'bdev->bd_invalidated' need to be set.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>