Fix race condition in image layer (#1440)

* Fix race condition in image layer

refer #1439

* Add explicit drop(inner) in layer load method

* Add explicit drop(inner) in layer load method

pageserver/src/layered_repository/image_layer.rs

@@ -267,7 +267,7 @@ impl ImageLayer {
             // a write lock. (Or rather, release and re-lock in write mode.)
             drop(inner);
             let mut inner = self.inner.write().unwrap();
-            if inner.book.is_none() {
+            if !inner.loaded {
                 self.load_inner(&mut inner)?;
             } else {
                 // Another thread loaded it while we were not holding the lock.