block: check disk exist before trying to add partition

If disk have been deleted, we should return fail for ioctl
BLKPG_DEL_PARTITION. Otherwise, the directory /sys/class/block
may remain invalid symlinks file. The race as following:

blkdev_open
				del_gendisk
				    disk->flags &= ~GENHD_FL_UP;
				    blk_drop_partitions
blkpg_ioctl
    bdev_add_partition
    add_partition
        device_add
	    device_add_class_symlinks

ioctl may add_partition after del_gendisk() have tried to delete
partitions. Then, symlinks file will be created.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Link: https://lore.kernel.org/r/20210610023241.3646241-1-yuyufen@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>