Changed ClusterStatsNodes.ProcessStats stream serialization logic to use readLong instead of readVLong for file descriptors

The minOpenFileDescriptors/maxOpenFileDescriptors can be negative valued if the ClusterStatsNodes response was initialized with no nodes.

Relates to elastic#4681

Author: bleskes

src/main/java/org/elasticsearch/action/admin/cluster/stats/ClusterStatsNodes.java

@@ -449,8 +449,8 @@ public void readFrom(StreamInput in) throws IOException {
             cpuPercent = in.readVInt();
             totalOpenFileDescriptors = in.readVLong();
             if (in.getVersion().onOrAfter(Version.V_0_90_10)) {
-                minOpenFileDescriptors = in.readVLong();
-                maxOpenFileDescriptors = in.readVLong();
+                minOpenFileDescriptors = in.readLong();
+                maxOpenFileDescriptors = in.readLong();
             }
         }
 
@@ -460,8 +460,8 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeVInt(cpuPercent);
             out.writeVLong(totalOpenFileDescriptors);
             if (out.getVersion().onOrAfter(Version.V_0_90_10)) {
-                out.writeVLong(minOpenFileDescriptors);
-                out.writeVLong(maxOpenFileDescriptors);
+                out.writeLong(minOpenFileDescriptors);
+                out.writeLong(maxOpenFileDescriptors);
             }
         }