info: Populate pool/set/disk indexes for offline disks (#18613)

This can be calculated from the disk layout and some external
applications would like to know the location of the offline
disks.

cmd/notification.go

@@ -977,6 +977,9 @@ func getOfflineDisks(offlineHost string, endpoints EndpointServerPools) []madmin
 				offlineDisks = append(offlineDisks, madmin.Disk{
 					Endpoint:  ep.String(),
 					State:     string(madmin.ItemOffline),
+					PoolIndex: ep.PoolIdx,
+					SetIndex:  ep.SetIdx,
+					DiskIndex: ep.DiskIdx,
 				})
 			}
 		}