mm/hmm: add missing pfns set to hmm_vma_walk_pmd()

All success exit paths from the walker functions must set the pfns array.

A migration entry with no required fault is a HMM_PFN_NONE return, just
like the pte case.

Fixes: d08faca018 ("mm/hmm: properly handle migration pmd")
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

mm/hmm.c

@@ -402,7 +402,7 @@ again:
 			pmd_migration_entry_wait(walk->mm, pmdp);
 			return -EBUSY;
 		}
-		return 0;
+		return hmm_pfns_fill(start, end, range, HMM_PFN_NONE);
 	} else if (!pmd_present(pmd))
 		return hmm_pfns_fill(start, end, range, HMM_PFN_ERROR);