CompositeByteBuf nioBuffer doesn't always alloc (netty#8176)

In nioBuffer(int,int) in CompositeByteBuf , we create a sub-array of nioBuffers for the components that are in range, then concatenate all the components in range into a single bigger buffer.
However, if the call to nioBuffers() returned only one sub-buffer, then we are copying it to a newly-allocated buffer "merged" for no reason.

Motivation:

Profiler for Spark shows a lot of time spent in put() method inside nioBuffer(), while usually no copy of data is required.

Modification:
This change skips this last step and just returns a duplicate of the single buffer returned by the call to nioBuffers(), which will in most implementation not copy the data

Result:
No copy when the source is only 1 buffer

buffer/src/main/java/io/netty/buffer/CompositeByteBuf.java

@@ -1467,9 +1467,13 @@ public ByteBuffer nioBuffer(int index, int length) {
             }
         }
 
-        ByteBuffer merged = ByteBuffer.allocate(length).order(order());
         ByteBuffer[] buffers = nioBuffers(index, length);
 
+        if (buffers.length == 1) {
+            return buffers[0].duplicate();
+        }
+
+        ByteBuffer merged = ByteBuffer.allocate(length).order(order());
         for (ByteBuffer buf: buffers) {
             merged.put(buf);
         }