block: fix that part scan is disabled in device_add_disk()

hulk inclusion
category: bugfix
bugzilla: 187190, https://gitee.com/src-openeuler/kernel/issues/I5GWOV


CVE: NA

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Commit f20a726b ("block: Fix warning in bd_link_disk_holder()")
moves the setting of flag 'GENHD_FL_UP' behind blkdev_get, which will
disabled part scan:

devcie_add_disk
 register_disk
  blkdev_get
   __blkdev_get
    bdev_get_gendisk
     get_gendisk -> failed because 'GENHD_FL_UP' is not set

And this will cause tests block/017, block/018 and scsi/004 to fail.

Fix the problem by moving part scan as well.

Fixes: f20a726b ("block: Fix warning in bd_link_disk_holder()")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>