Introduce Blockdev_Length/_Offset
diff --git a/src/filo-blockdev.adb b/src/filo-blockdev.adb
index 28747e7..ba7bc6d 100644
--- a/src/filo-blockdev.adb
+++ b/src/filo-blockdev.adb
@@ -2,7 +2,7 @@
 
    procedure Read
       (Buffer  : in out Buffer_Type;
-       Offset  : in     Natural;
+       Offset  : in     Blockdev_Offset;
        Success :    out Boolean)
    is
       use Interfaces.C;