atomic requires struct field to be 64-bit aligned

fix gohugoio/hugo#1621

Author: tpng

memfile.go

@@ -34,16 +34,16 @@ type MemDir interface {
 }
 
 type InMemoryFile struct {
-	sync.Mutex
 	at           int64
+	readDirCount int64
+	sync.Mutex
 	name         string
 	data         []byte
 	memDir       MemDir
 	dir          bool
 	closed       bool
 	mode         os.FileMode
 	modtime      time.Time
-	readDirCount int64
 }
 
 func MemFileCreate(name string) *InMemoryFile {