xl: Fix counting offline disks in StorageInfo (#9082)

Recent modification in the code led to incorrect calculation
of offline disks.

This commit saves the endpoint list in a xlObjects then we know
the name of each disk.

cmd/xl-sets.go

@@ -302,10 +302,16 @@ func newXLSets(endpoints Endpoints, format *formatXLV3, setCount int, drivesPerS
 		s.xlDisks[i] = make([]StorageAPI, drivesPerSet)
 		s.xlLockers[i] = make([]dsync.NetLocker, drivesPerSet)
 
+		var endpoints Endpoints
+		for j := 0; j < drivesPerSet; j++ {
+			endpoints = append(endpoints, s.endpoints[i*s.drivesPerSet+j])
+		}
+
 		// Initialize xl objects for a given set.
 		s.sets[i] = &xlObjects{
 			getDisks:    s.GetDisks(i),
 			getLockers:  s.GetLockers(i),
+			endpoints:   endpoints,
 			nsMutex:     mutex,
 			bp:          bp,
 			mrfUploadCh: make(chan partialUpload, 10000),