block: don't set GD_NEED_PART_SCAN if scan partition failed

Currently if disk_scan_partitions() failed, GD_NEED_PART_SCAN will still
set, and partition scan will be proceed again when blkdev_get_by_dev()
is called. However, this will cause a problem that re-assemble partitioned
raid device will creat partition for underlying disk.

Test procedure:

mdadm -CR /dev/md0 -l 1 -n 2 /dev/sda /dev/sdb -e 1.0
sgdisk -n 0:0:+100MiB /dev/md0
blockdev --rereadpt /dev/sda
blockdev --rereadpt /dev/sdb
mdadm -S /dev/md0
mdadm -A /dev/md0 /dev/sda /dev/sdb

Test result: underlying disk partition and raid partition can be
observed at the same time

Note that this can still happen in come corner cases that
GD_NEED_PART_SCAN can be set for underlying disk while re-assemble raid
device.

Fixes: e5cfefa9 ("block: fix scan partition for exclusively open device again")
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -368,7 +368,6 @@ int disk_scan_partitions(struct gendisk *disk, fmode_t mode)
 	if (disk->open_partitions)
 		return -EBUSY;
 
-	set_bit(GD_NEED_PART_SCAN, &disk->state);
 	/*
 	 * If the device is opened exclusively by current thread already, it's
 	 * safe to scan partitons, otherwise, use bd_prepare_to_claim() to
@@ -381,12 +380,19 @@ int disk_scan_partitions(struct gendisk *disk, fmode_t mode)
 			return ret;
 	}
 
+	set_bit(GD_NEED_PART_SCAN, &disk->state);
 	bdev = blkdev_get_by_dev(disk_devt(disk), mode & ~FMODE_EXCL, NULL);
 	if (IS_ERR(bdev))
 		ret =  PTR_ERR(bdev);
 	else
 		blkdev_put(bdev, mode & ~FMODE_EXCL);
 
+	/*
+	 * If blkdev_get_by_dev() failed early, GD_NEED_PART_SCAN is still set,
+	 * and this will cause that re-assemble partitioned raid device will
+	 * creat partition for underlying disk.
+	 */
+	clear_bit(GD_NEED_PART_SCAN, &disk->state);
 	if (!(mode & FMODE_EXCL))
 		bd_abort_claiming(disk->part0, disk_scan_partitions);
 	return ret;