nfs: bump max IO size to 16MB

From issue#145

```
JAVA's TCP offload support only kicks in if the buffer size is >= 4*default huge page size.
Default huge page size on x86-64 is 2MB, therefore at least a 8MB buffer is needed.
```

Fixes: #145
Acked-by: Paul Millar
Target: master

Author: kofemann

core/src/main/java/org/dcache/nfs/v4/NFSv4Defaults.java

@@ -31,7 +31,7 @@ public interface NFSv4Defaults {
     /**
      * maximal read/write buffer size.
      */
-    public static final long NFS4_MAXIOBUFFERSIZE = 1024*1024*4;
+    public static final long NFS4_MAXIOBUFFERSIZE = 1024*1024*16; // 16MB
 
     // theoretically, there is no limit on file size
     public final static long NFS4_MAXFILESIZE = Long.MAX_VALUE;