Fix volume_create breaking the build

weed/storage/volume_create.go

@@ -9,7 +9,7 @@ import (
 )
 
 func createVolumeFile(fileName string, preallocate int64, useMemoryMap uint32) (*os.File, error) {
-	file, e = os.OpenFile(fileName, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
+	file, e := os.OpenFile(fileName, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
 	if preallocate > 0 {
 		glog.V(0).Infof("Preallocated disk space for %s is not supported", fileName)
 	}