eth: use NewDB hook also for extra DB

(cherry picked from commit d5083033f1)

eth/backend.go

@@ -171,7 +171,10 @@ func New(config *Config) (*Ethereum, error) {
 	if err != nil {
 		return nil, err
 	}
-	extraDb, err := ethdb.NewLDBDatabase(path.Join(config.DataDir, "extra"))
+	extraDb, err := newdb(path.Join(config.DataDir, "extra"))
+	if err != nil {
+		return nil, err
+	}
 
 	// Perform database sanity checks
 	d, _ := blockDb.Get([]byte("ProtocolVersion"))