Forget to set error code when nd_label_alloc_slot failed, and we
add it to avoid overwritten error code.
Fixes: 
		
	
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|---|---|---|
| .. | ||
| badrange.c | ||
| blk.c | ||
| btt_devs.c | ||
| btt.c | ||
| btt.h | ||
| bus.c | ||
| claim.c | ||
| core.c | ||
| dax_devs.c | ||
| dimm_devs.c | ||
| dimm.c | ||
| e820.c | ||
| Kconfig | ||
| label.c | ||
| label.h | ||
| Makefile | ||
| namespace_devs.c | ||
| nd_virtio.c | ||
| nd-core.h | ||
| nd.h | ||
| of_pmem.c | ||
| pfn_devs.c | ||
| pfn.h | ||
| pmem.c | ||
| pmem.h | ||
| region_devs.c | ||
| region.c | ||
| security.c | ||
| virtio_pmem.c | ||
| virtio_pmem.h | ||