HBASE-23056 Block count is 0 when BucketCache using persistent IOEngine and retrieve from file

Signed-off-by: Reid Chan <reidchan@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketCache.java

@@ -1120,6 +1120,7 @@ public class BucketCache implements BlockCache, HeapSize {
       bucketAllocator = allocator;
       deserialiserMap = deserMap;
       backingMap = backingMapFromFile;
+      blockNumber.set(backingMap.size());
     } finally {
       if (ois != null) {
         ois.close();