NETTY-410 NioWorker write deadlock

Applied the patch provided by Greg Dhuse.

Author: trustin

src/main/java/org/jboss/netty/channel/socket/nio/NioDatagramWorker.java

@@ -568,15 +568,21 @@ private void write0(final NioDatagramChannel channel) {
                 }
             }
             channel.inWriteNowLoop = false;
+
+            // Initially, the following block was executed after releasing
+            // the writeLock, but there was a race condition, and it has to be
+            // executed before releasing the writeLock:
+            //
+            //     https://issues.jboss.org/browse/NETTY-410
+            //
+            if (addOpWrite) {
+                setOpWrite(channel);
+            } else if (removeOpWrite) {
+                clearOpWrite(channel);
+            }
         }
 
         fireWriteComplete(channel, writtenBytes);
-
-        if (addOpWrite) {
-            setOpWrite(channel);
-        } else if (removeOpWrite) {
-            clearOpWrite(channel);
-        }
     }
 
     private void setOpWrite(final NioDatagramChannel channel) {