Limit workaround on JDK-6427854 to Java <1.7 (netty#11863)

Motivation:

The workaround introduced by netty#203 (completed by netty#5644) on bug
[JDK-6427854](https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6427854)
turns out to be no longer applicable since Java 1.7.

Only first few builds of JDK 7 were affected by the bug, which got
fixed in **build 8**. Since JDK 7 was feature complete in
[build 123](https://blogs.oracle.com/java/post/jdk-7-feature-complete),
there's no need to therefore apply the workaround since Java 1.7.

Modifications:

This commit makes sure the workaround (consisting in setting the system
property `sun.nio.ch.bugLevel` to an empty string unless defined)
doesn't get applied when the detected Java version is greater than or
equal to 1.7.

Result:

The workaround gets only applied for Java versions strictly prior to 1.7.

Conditioning the workaround to the Java version will incidentally help
get rid of it when bumping up the minimum JDK support as proposed in
various issues s.a. netty#8259 and netty#8540.

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

@@ -76,22 +76,25 @@ public int get() throws Exception {
     // Workaround for JDK NIO bug.
     //
     // See:
-    // - https://bugs.java.com/view_bug.do?bug_id=6427854
+    // - https://bugs.openjdk.java.net/browse/JDK-6427854 for first few dev (unreleased) builds of JDK 7
+    // - https://bugs.openjdk.java.net/browse/JDK-6527572 for JDK prior to 5.0u15-rev and 6u10
     // - https://github.com/netty/netty/issues/203
     static {
-        final String key = "sun.nio.ch.bugLevel";
-        final String bugLevel = SystemPropertyUtil.get(key);
-        if (bugLevel == null) {
-            try {
-                AccessController.doPrivileged(new PrivilegedAction<Void>() {
-                    @Override
-                    public Void run() {
-                        System.setProperty(key, "");
-                        return null;
-                    }
-                });
-            } catch (final SecurityException e) {
-                logger.debug("Unable to get/set System Property: " + key, e);
+        if (PlatformDependent.javaVersion() < 7) {
+            final String key = "sun.nio.ch.bugLevel";
+            final String bugLevel = SystemPropertyUtil.get(key);
+            if (bugLevel == null) {
+                try {
+                    AccessController.doPrivileged(new PrivilegedAction<Void>() {
+                        @Override
+                        public Void run() {
+                            System.setProperty(key, "");
+                            return null;
+                        }
+                    });
+                } catch (final SecurityException e) {
+                    logger.debug("Unable to get/set System Property: " + key, e);
+                }
             }
         }