Drop `File_Len` from VFS.Read()

The `File_Len` is never supposed to change outside the FS driver.
diff --git a/src/filo-fs-vfs.adb b/src/filo-fs-vfs.adb
index 838cddf..a76d84c 100644
--- a/src/filo-fs-vfs.adb
+++ b/src/filo-fs-vfs.adb
@@ -54,7 +54,7 @@
       Read_Len : Natural;
    begin
       if Is_Open (State) then
-         Read (State, File_Max, File_Pos, Buffer, Read_Len);
+         Read (State, File_Pos, Buffer, Read_Len);
          Set_File_Pos (File_Pos);
       else
          Read_Len := 0;