xl: Avoid possible race during bulk Multi Delete (#7644)

errs was passed to many goroutines but they are all allowed
to update errs if any error happens during deletion, which
can cause a data race.

This commit will avoid issuing bulk delete operations in parallel
to avoid the warning race.

cmd/xl-v1-object.go

@@ -803,30 +803,22 @@ func (xl xlObjects) doDeleteObjects(ctx context.Context, bucket string, objects
 		}
 	}
 
-	// Initialize sync waitgroup.
-	var wg = &sync.WaitGroup{}
 	// Initialize list of errors.
 	var opErrs = make([]error, len(disks))
 	var delObjErrs = make([][]error, len(disks))
 
+	// Remove objects in bulk for each disk
 	for index, disk := range disks {
 		if disk == nil {
 			opErrs[index] = errDiskNotFound
 			continue
 		}
-		wg.Add(1)
-		go func(index int, disk StorageAPI) {
-			defer wg.Done()
 		delObjErrs[index], opErrs[index] = cleanupObjectsBulk(ctx, disk, minioMetaTmpBucket, tmpObjs, errs)
 		if opErrs[index] == errVolumeNotFound {
 			opErrs[index] = nil
 		}
-		}(index, disk)
 	}
 
-	// Wait for all routines to finish.
-	wg.Wait()
-
 	// Return errors if any during deletion
 	if err := reduceWriteQuorumErrs(ctx, opErrs, objectOpIgnoredErrs, len(xl.getDisks())/2+1); err != nil {
 		return nil, err