Fingerprint: add new signature method

commit c42a54795d introcuded a bug by
using fp.to_string(). Replace this with fp.signature() which correctly
returns the full fingerprint instead of the short version.

Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>

src/tape/drive/lto/mod.rs

@@ -317,7 +317,7 @@ impl TapeDriver for LtoTapeHandle {
         }
 
         let output = if let Some((fingerprint, uuid)) = key_fingerprint {
-            let fingerprint = fingerprint.to_string();
+            let fingerprint = fingerprint.signature();
             run_sg_tape_cmd("encryption", &[
                 "--fingerprint", &fingerprint,
                 "--uuid", &uuid.to_string(),