Do not consider PortUnreachableException to require channel closure

Motivation:

When a datagram packet is sent to a destination where nobody actually listens to,
the server O/S will respond with an ICMP Port Unreachable packet.
The ICMP Port Unreachable packet is translated into PortUnreachableException by JDK.
PortUnreachableException is not a harmful exception that prevents a user from sending a datagram.
Therefore, we should not close a datagram channel when PortUnreachableException is caught.

Modifications:

- Do not close a channel when the caught exception is PortUnreachableException.

Result:

A datagram channel is not closed unexpectedly anymore.

transport/src/main/java/io/netty/channel/nio/AbstractNioMessageChannel.java

@@ -22,6 +22,7 @@ import io.netty.channel.ChannelPipeline;
 import io.netty.channel.ServerChannel;
 
 import java.io.IOException;
+import java.net.PortUnreachableException;
 import java.nio.channels.SelectableChannel;
 import java.nio.channels.SelectionKey;
 import java.util.ArrayList;
@@ -96,7 +97,7 @@ public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
                 pipeline.fireChannelReadComplete();
 
                 if (exception != null) {
-                    if (exception instanceof IOException) {
+                    if (exception instanceof IOException && !(exception instanceof PortUnreachableException)) {
                         // ServerChannel should not be closed even on IOException because it can often continue
                         // accepting incoming connections. (e.g. too many open files)
                         closed = !(AbstractNioMessageChannel.this instanceof ServerChannel);