tape: clear encryption key before writing labels

We always write labels unencrypted.

src/tape/drive/linux_tape.rs

@@ -461,6 +461,8 @@ impl TapeDriver for LinuxTapeHandle {
             bail!("write_media_set_label failed - got wrong file number ({} != 1)", file_number);
         }
 
+        self.set_encryption(None)?;
+
         let mut handle = TapeWriterHandle {
             writer: BlockedWriter::new(&mut self.file),
         };

src/tape/drive/mod.rs

@@ -81,6 +81,8 @@ pub trait TapeDriver {
 
         self.rewind()?;
 
+        self.set_encryption(None)?;
+
         self.erase_media(true)?;
 
         let raw = serde_json::to_string_pretty(&serde_json::to_value(&label)?)?;

src/tape/drive/virtual_tape.rs

@@ -327,6 +327,8 @@ impl TapeDriver for VirtualTapeHandle {
 
     fn write_media_set_label(&mut self, media_set_label: &MediaSetLabel) -> Result<(), Error> {
 
+        self.set_encryption(None)?;
+
         let mut status = self.load_status()?;
         match status.current_tape {
             Some(VirtualTapeStatus { ref name, ref mut pos }) => {