usage: Fix buckets count calculation when no object is present (#8929)

XL crawling wrongly returns a zero buckets count when
there are no objects uploaded in the server yet. The reason is 
data of the crawler of posix returns invalid result when all 
disks has zero objects.

A simple fix is to always pick the crawling result of the first 
disk but choose over the result of the disk which has the most 
objects in it.

cmd/xl-v1.go

@@ -227,8 +227,10 @@ func (xl xlObjects) CrawlAndGetDataUsage(ctx context.Context, endCh <-chan struc
 	}
 	wg.Wait()
 
-	var dataUsageInfo DataUsageInfo
-	for i := 0; i < len(dataUsageResults); i++ {
+	var dataUsageInfo = dataUsageResults[0]
+	// Pick the crawling result of the disk which has the most
+	// number of objects in it.
+	for i := 1; i < len(dataUsageResults); i++ {
 		if dataUsageResults[i].ObjectsCount > dataUsageInfo.ObjectsCount {
 			dataUsageInfo = dataUsageResults[i]
 		}