media_catalog: use stream_position

Fixes the following clippy lint:

```
warning: using `SeekFrom::Current` to start from current position
   --> src/tape/media_catalog.rs:798:23
    |
798 |             let pos = file.seek(SeekFrom::Current(0))?; // get current pos
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `file.stream_position()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#seek_from_current
    = note: `#[warn(clippy::seek_from_current)]` on by default
```

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>

src/tape/media_catalog.rs

@@ -1,6 +1,6 @@
 use std::collections::{HashMap, HashSet};
 use std::fs::File;
-use std::io::{BufReader, Read, Seek, SeekFrom, Write};
+use std::io::{BufReader, Read, Seek, Write};
 use std::os::unix::io::AsRawFd;
 use std::path::{Path, PathBuf};
 
@@ -795,7 +795,7 @@ impl MediaCatalog {
         let mut media_set_uuid = None;
 
         loop {
-            let pos = file.seek(SeekFrom::Current(0))?; // get current pos
+            let pos = file.stream_position()?; // get current pos
 
             if pos == 0 {
                 // read/check magic number