Fix memory leak of direct memory. (#3983)

Descriptions of the changes in this PR:

### Motivation
When open a non-exist file for DirectReader, it will throw an exception. The ByteBuf won't release.
When close DirectWriter, if the close failed, the ByteBuf also won't release.

(cherry picked from commit c8c593e)

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/directentrylogger/DirectReader.java

@@ -56,8 +56,6 @@ class DirectReader implements LogReader {
         this.filename = filename;
         this.maxSaneEntrySize = maxSaneEntrySize;
         this.readBlockStats = readBlockStats;
-
-        nativeBuffer = new Buffer(nativeIO, bufferSize);
         closed = false;
 
         try {
@@ -71,6 +69,7 @@ class DirectReader implements LogReader {
                                   .kv("errno", ne.getErrno()).toString());
         }
         refreshMaxOffset();
+        nativeBuffer = new Buffer(nativeIO, bufferSize);
     }
 
     @Override

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/storage/directentrylogger/DirectWriter.java

@@ -233,10 +233,11 @@ public void close() throws IOException {
             throw new IOException(exMsg(ne.getMessage())
                                   .kv("file", filename)
                                   .kv("errno", ne.getErrno()).toString());
-        }
-        synchronized (bufferLock) {
-            bufferPool.release(nativeBuffer);
-            nativeBuffer = null;
+        } finally {
+            synchronized (bufferLock) {
+                bufferPool.release(nativeBuffer);
+                nativeBuffer = null;
+            }
         }
     }