block: fix an error code in add_partition()

We don't set an error code on this path.  It means that we return NULL
instead of an error pointer and the caller does a NULL dereference.

Fixes: 6d1d8050

 ("block, partition: add partition_meta_info to hd_struct")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>