devmapper: Move pool id query and migration of old data in separate function

Right now initMetaData() first queries the pool for current transaciton Id
and then it migrates the old metafile.

Move pool transaction Id query and file migration in separate functions
for better code reuse and organization.

Given we have removed device transaction Id dependency from saveMetaData(),
we don't have to query pool transaction Id before migrating files.
 
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -352,19 +352,8 @@ func (devices *DeviceSet) createFilesystem(info *DevInfo) error {
 	return nil
 }
 
-func (devices *DeviceSet) initMetaData() error {
+func (devices *DeviceSet) migrateOldMetaData() error {
-	_, _, _, params, err := devicemapper.GetStatus(devices.getPoolName())
-	if err != nil {
-		return err
-	}
-
-	if _, err := fmt.Sscanf(params, "%d", &devices.TransactionId); err != nil {
-		return err
-	}
-	devices.NewTransactionId = devices.TransactionId
-
 	// Migrate old metadatafile
-
 	jsonData, err := ioutil.ReadFile(devices.oldMetadataFile())
 	if err != nil && !os.IsNotExist(err) {
 		return err
@@ -390,6 +379,21 @@ func (devices *DeviceSet) initMetaData() error {
 	return nil
 }
 
+func (devices *DeviceSet) initMetaData() error {
+	if err := devices.migrateOldMetaData(); err != nil {
+		return err
+	}
+
+	_, transactionId, _, _, _, _, err := devices.poolStatus()
+	if err != nil {
+		return err
+	}
+
+	devices.TransactionId = transactionId
+	devices.NewTransactionId = devices.TransactionId
+	return nil
+}
+
 func (devices *DeviceSet) loadMetadata(hash string) *DevInfo {
 	info := &DevInfo{Hash: hash, devices: devices}