fix fillReadAheadCache stat bug

These code blocks will not be executed:
dbLedgerStorageStats.getReadAheadBatchCountStats().registerSuccessfulValue(count);
dbLedgerStorageStats.getReadAheadBatchSizeStats().registerSuccessfulValue(size);

### Motivation

fix fillReadAheadCache stat bug

### Changes

Replace return with break to exit the while loop



Reviewers: Enrico Olivelli <eolivelli@gmail.com>, Rajan Dhabalia <rdhabalia@apache.org>

This closes #2383 from holmes07/master
diff --git a/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/SingleDirectoryDbLedgerStorage.java b/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/SingleDirectoryDbLedgerStorage.java
index 60e3d64..931c3e0 100644
--- a/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/SingleDirectoryDbLedgerStorage.java
+++ b/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/ldb/SingleDirectoryDbLedgerStorage.java
@@ -475,7 +475,7 @@
 
                     if (currentEntryLedgerId != orginalLedgerId) {
                         // Found an entry belonging to a different ledger, stopping read-ahead
-                        return;
+                        break;
                     }
 
                     // Insert entry in read cache