Truncate waltmp file on creation (#8133)

Previously in safekeeper code, new segment file was opened without
truncate option. I don't think there is a reason to do it, this commit
replaces it with `File::create` to make it simpler and remove
`clippy::suspicious_open_options` linter warning.

safekeeper/src/wal_storage.rs

@@ -231,11 +231,7 @@ impl PhysicalStorage {
             // half initialized segment, first bake it under tmp filename and
             // then rename.
             let tmp_path = self.timeline_dir.join("waltmp");
-            #[allow(clippy::suspicious_open_options)]
-            let mut file = OpenOptions::new()
-                .create(true)
-                .write(true)
-                .open(&tmp_path)
+            let mut file = File::create(&tmp_path)
                 .await
                 .with_context(|| format!("Failed to open tmp wal file {:?}", &tmp_path))?;