MkdirAll should respect permissions (MemMapFS)

memmap.go

diff --git a/memmap.go b/memmap.go
index 7376c24..6824ab5 100644
--- a/memmap.go
+++ b/memmap.go
@@ -204,7 +204,7 @@
 }
 
 func (m *MemMapFs) MkdirAll(path string, perm os.FileMode) error {
-	return m.Mkdir(path, 0777)
+	return m.Mkdir(path, perm)
 }
 
 // Handle some relative paths