src/backup/data_blob.rs: define create_zero_chunk as member function

and rename it to build_zero_chunk()

src/backup/data_blob.rs

@@ -425,10 +425,9 @@ impl <'a, 'b> DataChunkBuilder<'a, 'b> {
 
         Ok((chunk, self.digest))
     }
-}
 
     /// Create a chunk filled with zeroes
-pub fn create_zero_chunk(
+    pub fn build_zero_chunk(
         crypt_config: Option<&CryptConfig>,
         chunk_size: usize,
         compress: bool,
@@ -443,3 +442,5 @@ pub fn create_zero_chunk(
 
         chunk_builder.build()
     }
+
+}