libnvdimm/label: Return -ENXIO for no slot in __blk_label_update

Forget to set error code when nd_label_alloc_slot failed, and we
add it to avoid overwritten error code.

Fixes: 0ba1c634

 ("libnvdimm: write blk label set")
Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Link: https://lore.kernel.org/r/20201205115056.2076523-1-zhangqilong3@huawei.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>