fix: background heal rely on getOnlineDisks() (#10687)

cmd/erasure-sets.go

@@ -1026,6 +1026,7 @@ func (s *erasureSets) startMergeWalksVersionsN(ctx context.Context, bucket, pref
 			wg.Add(1)
 			go func(disk StorageAPI) {
 				defer wg.Done()
+
 				entryCh, err := disk.WalkVersions(GlobalContext, bucket, prefix, marker, recursive, endWalkCh)
 				if err != nil {
 					return