Better reporting of total/free usable capacity of the cluster (#15230)

The current code uses approximation using a ratio. The approximation 
can skew if we have multiple pools with different disk capacities.

Replace the algorithm with a simpler one which counts data 
disks and ignore parity disks.

cmd/fs-v1.go

@@ -209,7 +209,13 @@ func (fs *FSObjects) Shutdown(ctx context.Context) error {
 
 // BackendInfo - returns backend information
 func (fs *FSObjects) BackendInfo() madmin.BackendInfo {
-	return madmin.BackendInfo{Type: madmin.FS}
+	return madmin.BackendInfo{
+		Type:             madmin.FS,
+		StandardSCData:   []int{1},
+		StandardSCParity: 0,
+		RRSCData:         []int{1},
+		RRSCParity:       0,
+	}
 }
 
 // LocalStorageInfo - returns underlying storage statistics.
@@ -234,7 +240,7 @@ func (fs *FSObjects) StorageInfo(ctx context.Context) (StorageInfo, []error) {
 			},
 		},
 	}
-	storageInfo.Backend.Type = madmin.FS
+	storageInfo.Backend = fs.BackendInfo()
 	return storageInfo, nil
 }