Work around gnatprove trouble with >64-bit range type
diff --git a/src/filo-blockdev.adb b/src/filo-blockdev.adb
index 8658e05..5bb6732 100644
--- a/src/filo-blockdev.adb
+++ b/src/filo-blockdev.adb
@@ -19,7 +19,7 @@
    begin
       -- With 32-bit longs, the current C interface
       -- can't access more than 512 * 2 ** 32:
-      if Offset / BLOCK_SIZE > Blockdev_Length (unsigned_long'last) then
+      if Unsigned_64 (Offset / BLOCK_SIZE) > Unsigned_64 (unsigned_long'last) then
          Success := False;
          return;
       end if;