Latest clippy has spoken!

Fix the following error reported by clippy:
```
error: manual arithmetic check found
  --> puzzlefs-lib/src/reader/puzzlefs.rs:45:27
   |
45 |           let addl_offset = if offset > file_offset {
   |  ___________________________^
46 | |             offset - file_offset
47 | |         } else {
48 | |             0
49 | |         };
   | |_________^ help: replace it with: `offset.saturating_sub(file_offset)`
      |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implicit_saturating_sub
   = note: `-D clippy::implicit-saturating-sub` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::implicit_saturating_sub)]`
```

Signed-off-by: Ariel Miculas-Trif <[email protected]>

puzzlefs-lib/src/reader/puzzlefs.rs

@@ -42,11 +42,7 @@ pub(crate) fn file_read(
             continue;
         }
 
-        let addl_offset = if offset > file_offset {
-            offset - file_offset
-        } else {
-            0
-        };
+        let addl_offset = offset.saturating_sub(file_offset);
 
         // ok, need to read this chunk; how much?
         let left_in_buf = data.len() - buf_offset;