fix mutex

box.go

@@ -78,9 +78,6 @@ func (b *Box) ZFSIsLastSnapshot(src Snapshot) (is bool, err error) {
 		return
 	}
 
-	b.zfs.M.Lock()
-	defer b.zfs.M.Unlock()
-
 	_, err = b.ZFSGetNextSnapshot(src)
 	if err != nil {
 		if err.Error() == "no snapshot" {