[#2469] Correctly update indices after incomplete gathering write

Motivation:
 When doing a gathering write we need to update the indices after the write partial completes. In the current code-base we use the wrong value when compare the expected written bytes and the actual written bytes.

 Modifications:
 Use the correct value when compare.

Result:
Indices are updated correctly and so no corruption can happen when resume writing after data was only partial written before.

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollSocketChannel.java

@@ -171,7 +171,7 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
             NioSocketChannelOutboundBuffer in, int msgCount, ByteBuffer[] nioBuffers) throws IOException {
 
         int nioBufferCnt = in.nioBufferCount();
-        long expectedWrittenBytes = in.nioBufferCount();
+        long expectedWrittenBytes = in.nioBufferSize();
 
         long localWrittenBytes = Native.writev(fd, nioBuffers, 0, nioBufferCnt);